arXiv subjects

Mike Lockwood

Publications and source records attributed to Mike Lockwood.

At least 19 recordsLinked to original sources

arXiv2025

UK White Paper on Magnetic Reconnection

Magnetic reconnection powers explosive releases of magnetic energy, heating and particle acceleration throughout the plasma universe. Knowledge of this universal process is vital to understanding the Heliosphere, as it plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, geomagnetic storms, and interactions between the solar wind and planetary magnetospheres. As such, reconnection underpins multiple science objectives of multiple future space missions. The UK plays a leading role in this international field, through a combination of in situ measurements from Earth's magnetosphere and the solar wind, observations of the solar corona and chromosphere, and world-class numerical simulations and theory. This white paper identifies: Nine priority science objectives for reconnection research in the next decade; Recommendations to guide investment in theory, simulations and infrastructure; Mission priorities and required measurements to ensure the UK maintains and improves its world-class credentials in reconnection science.

astro-ph.IM↗

arXiv2023

Slow Solar Wind Connection Science during Solar Orbiter's First Close Perihelion Passage

The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilise the extensive suite of remote sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote sensing and in situ measurements of slow wind originating at open-closed field boundaries. The SOOP ran just prior to Solar Orbiter's first close perihelion passage during two remote sensing windows (RSW1 and RSW2) between 2022 March 3-6 and 2022 March 17-22, while Solar Orbiter was at a heliocentric distance of 0.55-0.51 and 0.38-0.34 au from the Sun, respectively. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low latency in situ data, and full-disk remote sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Post-observation analysis using the magnetic connectivity tool along with in situ measurements from MAG and SWA/PAS, show that slow solar wind, with velocities between 210 and 600 km/s, arrived at the spacecraft originating from two out of the three of the target regions. The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

astro-ph.SR↗

arXiv2022

SIR-HUXt -- a particle filter data assimilation scheme for assimilating CME time-elongation profiles

We present the development of SIR-HUXt, the integration of a sequential importance resampling (SIR) data assimilation scheme with the HUXt solar wind model. SIR-HUXt is designed to assimilate the time-elongation profiles of CME fronts in the low heliosphere, such as those typically extracted from heliospheric imager data returned by the STEREO, Parker Solar Probe, and Solar Orbiter missions. We use Observing System Simulation Experiments to explore the performance of SIR-HUXt for a simple synthetic CME scenario of a fully Earth directed CME flowing through a uniform ambient solar wind, where the CME is initialised with the average observed CME speed and width. These experiments are performed for a range of observer locations, from 20 deg to 90 deg behind Earth, spanning the L5 point where ESA's future Vigil space weather monitor will return heliospheric imager data for operational space weather forecasting. We show that SIR-HUXt performs well at constraining the CME speed, and has some success at constraining the CME longitude. The CME width is largely unconstrained by the SIR-HUXt assimilations, and more experiments are required to determine if this is due to this specific CME scenario, or is a general feature of assimilating time-elongation profiles. Rank-histograms suggest that the SIR-HUXt ensembles are well calibrated, with no clear indications of bias or under/over dispersion. Improved constraints on the initial CME speed lead directly to improvements in the CME transit time to Earth and arrival speed. For an observer in the L5 region, SIR-HUXt returned a 69% reduction in the CME transit time uncertainty, and a 63% reduction in the arrival speed uncertainty. This suggests SIR-HUXt has potential to improve the real-world representivity of HUXt simulations, and therefore has potential to reduce the uncertainty of CME arrival time hindcasts and forecasts.

astro-ph.SR↗

arXiv2022

Solar Energetic Particle Ground-Level Enhancements and the Solar Cycle

Severe geomagnetic storms appear to be ordered by the solar cycle in a number of ways. They occur more frequently close to solar maximum and declining phase, are more common in larger solar cycles and show different patterns of occurrence in odd- and even-numbered solar cycles. Our knowledge of the most extreme space weather events, however, comes from the spikes in cosmogenic-isotope ($^{14}$C, $^{10}$Be and $^{36}$Cl) records that are attributed to significantly larger solar energetic particle (SEP) events than have been observed during the space age. Despite both storms and SEPs being driven by solar eruptive phenomena, the event-by-event correspondence between extreme storms and extreme SEPs is low. Thus it should not be assumed a priori that the solar cycle patterns found for storms also hold for SEPs and the cosmogenic-isotope events. In this study we investigate the solar cycle trends in the timing and magnitude of the 67 SEP ground-level enhancements (GLEs) recorded by neutron monitors since the mid 1950s. Using a number of models of GLE occurrence probability, we show that GLEs are around a factor four more likely around solar maximum than around solar minimum, and that they preferentially occur earlier in even-numbered solar cycles than in odd-numbered cycles. There are insufficient data to conclusively determine whether larger solar cycles produce more GLEs. Implications for putative space-weather events in the cosmogenic-isotope records are discussed. We find that GLEs tend to cluster within a few tens of days, likely due to particularly productive individual active regions, and with approximately 11-year separations, owing to the solar cycle ordering. But these timescales do not explain cosmogenic-isotope spikes which require multiple extreme SEP events over consecutive years.

astro-ph.SR↗

arXiv2021

Quantifying the uncertainty in CME kinematics derived from geometric modelling of Heliospheric Imager data

Geometric modelling of Coronal Mass Ejections (CMEs) is a widely used tool for assessing their kinematic evolution. Furthermore, techniques based on geometric modelling, such as ELEvoHI, are being developed into forecast tools for space weather prediction. These models assume that solar wind structure does not affect the evolution of the CME, which is an unquantified source of uncertainty. We use a large number of Cone CME simulations with the HUXt solar wind model to quantify the scale of uncertainty introduced into geometric modelling and the ELEvoHI CME arrival times by solar wind structure. We produce a database of simulations, representing an average, a fast, and an extreme CME scenario, each independently propagating through 100 different ambient solar wind environments. Synthetic heliospheric imager observations of these simulations are then used with a range of geometric models to estimate the CME kinematics. The errors of geometric modelling depend on the location of the observer, but do not seem to depend on the CME scenario. In general, geometric models are biased towards predicting CME apex distances that are larger than the true value. For these CME scenarios, geometric modelling errors are minimised for an observer in the L5 region. Furthermore, geometric modelling errors increase with the level of solar wind structure in the path of the CME. The ELEvoHI arrival time errors are minimised for an observer in the L5 region, with mean absolute arrival time errors of $8.2\pm1.2$~h, $8.3\pm1.0$~h, and $5.8\pm0.9$~h for the average, fast, and extreme CME scenarios.

astro-ph.SR↗

arXiv2021

Cosmic Meteorology

Mike Lockwood and Mathew Owens discuss how eclipse observations are aiding the development of a climatology of near-Earth space

physics.hist-ph↗

arXiv2021

Evolving Solar Wind Flow Properties of Magnetic Inversions Observed by Helios

In its first encounter at solar distances as close as r = 0.16AU, Parker Solar Probe (PSP) observed numerous local reversals, or inversions, in the heliospheric magnetic field (HMF), which were accompanied by large spikes in solar wind speed. Both solar and in situ mechanisms have been suggested to explain the existence of HMF inversions in general. Previous work using Helios 1, covering 0.3-1AU, observed inverted HMF to become more common with increasing r, suggesting that some heliospheric driving process creates or amplifies inversions. This study expands upon these findings, by analysing inversion-associated changes in plasma properties for the same large data set, facilitated by observations of 'strahl' electrons to identify the unperturbed magnetic polarity. We find that many inversions exhibit anti-correlated field and velocity perturbations, and are thus characteristically Alfv\'enic, but many also depart strongly from this relationship over an apparent continuum of properties. Inversions depart further from the 'ideal' Alfv\'enic case with increasing r, as more energy is partitioned in the field, rather than the plasma, component of the perturbation. This departure is greatest for inversions with larger density and magnetic field strength changes, and characteristic slow solar wind properties. We find no evidence that inversions which stray further from 'ideal' Alfv\'enicity have different generation processes from those which are more Alfv\'enic. Instead, different inversion properties could be imprinted based on transport or formation within different solar wind streams.

astro-ph.SR↗

arXiv2020

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 4. Polar Cap motions and origins of the Universal Time effect

We use the am, an, as and the a-sigma geomagnetic indices to the explore a previously overlooked factor in magnetospheric electrodynamics, namely the inductive effect of diurnal motions of the Earth's magnetic poles toward and away from the Sun caused by Earth's rotation. Because the offset of the (eccentric dipole) geomagnetic pole from the rotational axis is roughly twice as large in the southern hemisphere compared to the northern, the effects there are predicted to be roughly twice the amplitude. Hemispheric differences have previously been discussed in terms of polar ionospheric conductivities, effects which we allow for by studying the dipole tilt effect on time-of-year variations of the indices. The electric field induced in a geocentric frame is shown to also be a significant factor and gives a modulation of the voltage applied by the solar wind flow in the southern hemisphere of typically a 30% diurnal modulation for disturbed intervals rising to 76% in quiet times. Motion towards/away from the Sun reduces/enhances the directly-driven ionospheric voltages and reduces/enhances the magnetic energy stored in the near-Earth tail: 10% of the effect being directly-driven and 90% being in tail energy storage/release. Combined with the effect of solar wind dynamic pressure and dipole tilt on the pressure balance in the near-Earth tail, the effect provides an excellent explanation of how the observed Russell-McPherron pattern in the driving power input into the magnetosphere is converted into the equinoctial pattern in average geomagnetic activity (after correction is made for dipole tilt effects on ionospheric conductivity), added to a pronounced UT variation with minimum at 02-10UT. In addition, we show that the predicted and observed UT variations in average geomagnetic activity has implications for the occurrence of the largest events that also show the nett UT variation.

physics.space-ph↗

arXiv2020

The Evolution of Inverted Magnetic Fields Through the Inner Heliosphere

Local inversions are often observed in the heliospheric magnetic field (HMF), but their origins and evolution are not yet fully understood.Parker Solar Probe has recently observed rapid, Alfvenic, HMF inversions in the inner heliosphere, known as 'switchbacks', which have been interpreted as the possible remnants of coronal jets. It has also been suggested that inverted HMF may be produced by near-Sun interchange reconnection; a key process in mechanisms proposed for slow solar wind release. These cases suggest that the source of inverted HMF is near the Sun, and it follows that these inversions would gradually decay and straighten as they propagate out through the heliosphere. Alternatively, HMF inversions could form during solar wind transit, through phenomena such velocity shears, draping over ejecta, or waves and turbulence. Such processes are expected to lead to a qualitatively radial evolution of inverted HMF structures. Using Helios measurements spanning 0.3-1 AU, we examine the occurrence rate of inverted HMF, as well as other magnetic field morphologies, as a function of radial distance A, and find that it continually increases. This trend may be explained by inverted HMF observed between 0.3-1 AU being primarily driven by one or more of the above in-transit processes, rather than created at the Sun. We make suggestions as to the relative importance of these different processes based on the evolution of the magnetic field properties associated with inverted HMF. We also explore alternative explanations outside of our suggested driving processes which may lead to the observed trend.

physics.space-ph↗

arXiv2019

On the Origin of Otho-Gardenhose Heliospheric Flux

Parker spiral theory predicts the heliospheric magnetic field (HMF) will have components of opposite polarity radially toward the Sun and tangentially antiparallel to the solar rotation (i.e. in GSE coordinates, with Bx/By<0). This theory explains the average orientation of the HMF very well but does not predict orthogardenhose (OGH) flux with BX/BY>0 that is often observed. We here study the occurrence and structure of OGH flux, as seen in near-Earth space (r=1AU) and by the Helios spacecraft at 0.29<r<1AU, in order to evaluate the contributions to OGH flux generation of the various mechanisms and factors that are not accounted for in Parker spiral theory. We study the loss of OGH flux with increasing averaging timescale between 16 seconds and 100 hours and so determine its spectrum of scale sizes. OGH flux at sunspot minimum is shown to be more common than at sunspot maximum and caused by smaller-scale structure in the HMF, with a mode temporal scale at a fixed point of 10hrs compared to 40hrs. OGH generated by rotating the HMF through the radial direction is also shown to differ in its spectrum of scale sizes from that generated by rotating the HMF through the tangential direction: the former does not contribute to the excess open heliospheric flux at a given r but the latter does. We show that roughly half of the HMF deflection from the ideal Parker spiral giving OGH at Earth occurs at r below 0.3AU. We highlight some questions which can be addressed by the Parker Solar Probe mission which will study the HMF down to r=0.046AU.

physics.space-ph↗

arXiv2018

A homogeneous aa index: 1. Secular variation

Originally complied for 1868-1967 and subsequently continued so that it now covers 150 years, the $aa$ index has become a vital resource for studying space climate change. However, there have been debates about the inter-calibration of data from the different stations. In addition, the effects of secular change in the geomagnetic field have not previously been allowed for. As a result, the components of the 'classical' $aa$ index for the southern and northern hemispheres ($aa_S$ and $aa_N$) have drifted apart. We here separately correct both $aa_S$ and $aa_N$ for both these effects using the same method as used to generate the classic $aa$ values but allowing ${\delta}$, the minimum angular separation of each station from a nominal auroral oval, to vary as calculated using the IGRF-12 and gufm1 models of the intrinsic geomagnetic field. Our approach is to correct the quantized aK-values for each station, originally scaled on the assumption that ${\delta}$ values are constant, with time-dependent scale factors that allow for the drift in ${\delta}$. This requires revisiting the intercalibration of successive stations used in making the $aa_S$ and $aa_N$ composites. These intercalibrations are defined using independent data and daily averages from 11 years before and after each station change and it is shown that they depend on the time of year. This procedure produces new homogenized hemispheric aa indices, $aa_{HS}$ and $aa_{HN}$, which show centennial-scale changes that are in very close agreement. Calibration problems with the classic $aa$ index are shown to have arisen from drifts in ${\delta}$ combined with simpler corrections which gave an incorrect temporal variation and underestimate the rise in $aa$ during the 20th century by about 15%.

astro-ph.SR↗

arXiv2018

A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation

Paper 1 [Lockwood et al., 2018] generated annual means of a new version of the $aa$ geomagnetic activity index which includes corrections for secular drift in the geographic coordinates of the auroral oval, thereby resolving the difference between the centennial-scale change in the northern and southern hemisphere indices, $aa_N$ and $aa_S$. However, other hemispheric asymmetries in the $aa$ index remain: in particular, the distributions of 3-hourly $aa_N$ and $aa_S$ values are different and the correlation between them is not high on this timescale ($r = 0.66$). In the present paper, a location-dependant station sensitivity model is developed using the $am$ index (derived from a much more extensive network of stations in both hemispheres) and used to reduce the difference between the hemispheric $aa$ indices and improve their correlation (to $r = 0.79$) by generating corrected 3-hourly hemispheric indices, $aa_{HN}$ and $aa_{HS}$, which also include the secular drift corrections detailed in Paper 1. These are combined into a new, 'homogeneous' $aa$ index, $aa_H$. It is shown that $aa_H$, unlike $aa$, reveals the 'equinoctial'-like time-of-day/time-of-year pattern that is found for the $am$ index.

physics.space-ph↗

arXiv2018

Space climate and space weather over the past 400 years: 2. Proxy indicators of geomagnetic storm and substorm

Using the reconstruction of power input to the magnetosphere given in Paper 1 (arXiv:1708.04904), we reconstruct annual means of geomagnetic indices over the past 400 years to within a 1-sigma error of +/-20 pc. In addition, we study the behaviour of the lognormal distribution of daily and hourly values about these annual means and show that we can also reconstruct the fraction of geomagnetically-active (storm-like) days and (substorm-like) hours in each year to accuracies of 50-60 pc. The results are the first physics-based quantification of the space weather conditions in both the Dalton and Maunder minima. We predict terrestrial disturbance levels in future repeats of these minima, allowing for the weakening of Earth's dipole moment.

astro-ph.EP↗

arXiv2017

Space Climate and Space Weather over the past 400 years: 1. The Power input to the Magnetosphere

Using information on geomagnetic activity, sunspot numbers and cosmogenic isotopes, supported by historic eclipse images and in conjunction with models, it has been possible to reconstruct annual means of solar wind speed and number density and heliospheric magnetic field (HMF) intensity since 1611, when telescopic observations of sunspots began. These models are developed and tuned using data recorded by near-Earth interplanetary spacecraft and by solar magnetograms over the past 53 years. In this paper, we use these reconstructions to quantify power input into the magnetosphere over the past 400 years. For each year, both the annual mean power input is computed and its distribution in daily means. This is possible because the distribution of daily values divided by the annual mean is shown to maintain the same lognormal form with a constant variance. This study is another important step towards the development of a physics-based, long-term climatology of space weather conditions.

physics.space-ph↗

arXiv2016

Tests of sunspot number sequences: 4. Discontinuities around 1946 in various sunspot number and sunspot group number reconstructions

We use 5 test data series to quantify putative discontinuities around 1946 in 5 annual-mean sunspot number or group number sequences. The series tested are: the original and new versions of the Wolf/Zurich/International sunspot number composite [$R_{ISNv1}$ and $R_{ISNv2}$] ; the corrected version of $R_{ISNv1}$ [$R_C$]; the backbone group number [$R_{BB}$]; and the group number composite [$R_{UEA}$]. The test data are: the group number $N_G$ and total sunspot area $A_G$ from the RGO photoheliographic data; the CaK index from re-analysis of MWO CaII K spectroheliograms; the group number from the MWO sunspot drawings, $N_{MWO}$; and ionospheric critical frequencies at Slough [$foF2$]. The test data all vary with sunspot numbers, in some cases non-linearly. Tests use both before-and-after fit-residual comparison and correlation methods, applied to intervals iterated to minimise errors and eliminate the effect of the discontinuity date. It is not assumed that the correction required is by a constant factor, nor even linear in sunspot number. A non-linear correction is required by $R_C$, $R_{BB}$ and $R_{ISNv1}$, but not by $R_{ISNv2}$ or $R_{UEA}$. The test datasets give very similar results in all cases. By multiplying the probability distribution functions together we obtain the optimum correction for each data series that must be applied to pre-discontinuity data to make them consistent with the post-discontinuity data. It is shown that, on average, values for 1932-1943 are too small (relative to later values) by about 12.3% for $R_{ISNv1}$ but are too large for $R_{ISNv2}$ and $R_{BB}$ by 3.8% and 5.2%. The correction applied to generate $R_C$ from $R_{ISNv1}$ reduces this average factor to 0.5% but does not remove the non-linear variation, and other errors remain uncorrected. A test is provided by $R_{UEA}$, which is identical to the RGO $N_G$ values over the interval used.

astro-ph.SR↗

arXiv2016

Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data

We compare four sunspot-number data sequences against geomagnetic and terrestrial auroral observations. The comparisons are made for the original SIDC composite of Wolf-Zurich-International sunspot number [$R_{ISNv1}$], the group sunspot number [$R_{G}$] by Hoyt and Schatten (Solar Phys., 1998), the new "backbone" group sunspot number [$R_{BB}$] by Svalgaard and Schatten (Solar Phys., 2016), and the "corrected" sunspot number [$R_{C}$] by Lockwood at al. (J.G.R., 2014). Each sunspot number is fitted with terrestrial observations, or parameters derived from terrestrial observations to be linearly proportional to sunspot number, over a 30-year calibration interval of 1982-2012. The fits are then used to compute test sequences, which extend further back in time and which are compared to $R_{ISNv1}$, $R_{G}$, $R_{BB}$, and $R_{C}$. To study the long-term trends, comparisons are made using averages over whole solar cycles (minimum-to-minimum). The test variations are generated in four ways: i) using the IDV(1d) and IDV geomagnetic indices (for 1845-2013) fitted over the calibration interval using the various sunspot numbers and the phase of the solar cycle; ii) from the open solar flux (OSF) generated for 1845 - 2013 from four pairings of geomagnetic indices by Lockwood et al. (Ann. Geophys., 2014) and analysed using the OSF continuity model of Solanki at al. (Nature, 2000) which employs a constant fractional OSF loss rate; iii) the same OSF data analysed using the OSF continuity model of Owens and Lockwood (J.G.R., 2012) in which the fractional loss rate varies with the tilt of the heliospheric current sheet and hence with the phase of the solar cycle; iv) the occurrence frequency of low-latitude aurora for 1780-1980 from the survey of Legrand and Simon (Ann. Geophys., 1987). For all cases, $R_{BB}$ exceeds the test terrestrial series by an amount that increases as one goes back in time.

astro-ph.SR↗

arXiv2016

An Assessment of Sunspot Number Data Composites over 1845-2014

New sunspot data composites, some of which are radically different in the character of their long-term variation, are evaluated over the interval 1845-2014. The method commonly used to calibrate historic sunspot data, relative to modern-day data, is "daisy-chaining", whereby calibration is passed from one data subset to the neighbouring one, usually using regressions of the data subsets for the intervals of their overlap. Recent studies have illustrated serious pitfalls in these regressions and the resulting errors can be compounded by their repeated use as the data sequence is extended back in time. Hence the recent composite data series by Usoskin et al. (2016), $R_{UEA}$, is a very important advance because it avoids regressions, daisy-chaining and other common, but invalid, assumptions: this is achieved by comparing the statistics of "active day" fractions to those for a single reference dataset. We study six sunspot data series including $R_{UEA}$ and the new "backbone" data series $R_{BB}$, recently generated by Svalgaard and Schatten (2016) by employing both regression and daisy-chaining. We show that all six can be used with a continuity model to reproduce the main features of the open solar flux variation for 1845-2014, as reconstructed from geomagnetic activity data. However, some differences can be identified that are consistent with tests using a basket of other proxies for solar magnetic fields. Using data from a variety of sunspot observers, we illustrate problems with the method employed in $R_{BB}$ which cause it to increasingly overestimate sunspot numbers going back in time and we recommend using $R_{UEA}$ because it employs more robust procedures that avoid such problems.

astro-ph.SR↗

arXiv2015

Further considerations of cosmic ray modulation of infra-red radiation in the atmosphere

Understanding effects of ionisation in the lower atmosphere is a new interdisciplinary area, crossing traditionally distinct scientific boundaries. Following the paper of Erlykin et al. (Astropart. Phys. 57--58 (2014) 26--29) we develop the interpretation of observed changes in long-wave (LW) radiation (Aplin and Lockwood, Env. Res. Letts. 8, 015026 (2013)), by taking account of cosmic ray ionisation yields and atmospheric radiative transfer. To demonstrate this, we show that the thermal structure of the whole atmosphere needs to be considered along with the vertical profile of ionisation. Allowing for ionisation by all components of a cosmic ray shower and not just by the muons, reveals that the effect we have detected is certainly not inconsistent with laboratory observations of the LW absorption cross section. The analysis presented here, although very different from that of Erlykin et al., does come to the same conclusion that the events detected were not caused by individual cosmic ray primaries -- not because it is impossible on energetic grounds, but because events of the required energy are too infrequent for the 12/hr rate at which they were seen by the AL experiment. The present paper numerically models the effect of three different scenario changes to the primary GCR spectrum which all reproduce the required magnitude of the observed effect. However, they cannot solely explain the observed delay in the peak effect which, if confirmed, would appear to open up a whole new and interesting area in the study of water oligomers and their effects on LW radiation. We argue that a technical artefact in the earlier experiment is highly unlikely and that our initial observations merit both follow-up experiments and more rigorous, self-consistent, three-dimensional radiative transfer modelling.

physics.ao-ph↗