arXiv subjects

M. Temmer

Publications and source records attributed to M. Temmer.

At least 37 records · Page 2Linked to original sources

arXiv2020

Magnetic Flux Emergence in a Coronal Hole

A joint campaign of various space-borne and ground-based observatories, comprising the Japanese Hinode mission (HOP~338, 20\,--\,30~September 2017), the GREGOR solar telescope, and the \textit{Vacuum Tower Telescope} (VTT), investigated numerous targets such as pores, sunspots, and coronal holes. In this study, we focus on the coronal hole region target. On 24~September 2017, a very extended non-polar coronal hole developed patches of flux emergence, which contributed to the decrease of the overall area of the coronal hole. These flux emergence patches erode the coronal hole and transform the area into a more quiet-Sun-like area, whereby bipolar magnetic structures play an important role. Conversely, flux cancellation leads to the reduction of opposite-polarity magnetic fields and to an increase in the area of the coronal hole. Other global coronal hole characteristics, including the evolution of the associated magnetic flux and the aforementioned area evolution in the EUV, are studied using data of the \textit{Helioseismic and Magnetic Imager} (HMI) and \textit{Atmospheric Imaging Assembly} (AIA) onboard the \textit{Solar Dynamics Observatory} (SDO). The interplanetary medium parameters of the solar wind display parameters compatible with the presence of the coronal hole. Furthermore, a particular transient is found in those parameters.

astro-ph.SR↗

arXiv2020

A new method for estimating global coronal wave properties from their interaction with solar coronal holes

Global coronal waves (CWs) and their interaction with coronal holes (CHs) result, among other effects, in the formation of reflected and transmitted waves. Observations of such events provide us with measurements of different CW parameters, such as phase speed and intensity amplitudes. However, several of these parameters are provided with only intermediate observational quality, other parameters, such as the phase speed of transmitted waves, can hardly be observed in general. We present a new method to estimate crucial CW parameters, such as density and phase speed of reflected as well as transmitted waves, Mach numbers and density values of the CH's interior, by using analytical expressions in combination with basic and most accessible observational measurements. The transmission and reflection coefficients are derived from linear theory and subsequently used to calculate estimations for phase speeds of incoming, reflected and transmitted waves. The obtained analytical expressions are validated by performing numerical simulations of CWs interacting with CHs. This new method enables to determine in a fast and straightforward way reliable CW and CH parameters from basic observational measurements which provides a powerful tool to better understand the observed interaction effects between CWs and CHs.

astro-ph.SR↗

arXiv2019

Testing the background solar wind modelled by EUHFORIA

In order to address the growing need for more accurate space weather predictions, a new model named EUHFORIA (EUropean Heliospheric FORecasting Information Asset) was recently developed (Pomoell and Poedts, 2018). We present first results of the performance assessment for the solar wind modeling with EUHFORIA and identify possible limitations of its present setup. Using the basic EUHFORIA 1.0.4. model setup with the default input parameters, we modeled background solar wind (no coronal mass ejections) and compared the obtained results with ACE, in situ measurements. For the need of statistical study we developed a technique of combining daily EUHFORIA runs into continuous time series. The combined time series were derived for the years 2008 (low solar activity) and 2012 (high solar activity) from which in situ speed and density profiles were extracted. We find for the low activity phase a better match between model results and observations compared to the considered high activity time interval. The quality of the modeled solar wind parameters is found to be rather variable. Therefore, to better understand the obtained results we also qualitatively inspected characteristics of coronal holes, sources of the studied fast streams. We discuss how different characteristics of the coronal holes and input parameters to EUHFORIA influence the modeled fast solar wind, and suggest possibilities for the improvements of the model.

astro-ph.SR↗

arXiv2019

3-Phase Evolution of a Coronal Hole, Part II: The Magnetic Field

We investigate the magnetic characteristics of a persistent coronal hole (CH) extracted from EUV imagery using HMI filtergrams over the timerange February 2012-October 2012. The magnetic field, its distribution as well as the magnetic fine structure in form of flux tubes (FT) are analyzed in different evolutionary states of the CH. We find a strong linear correlation between the magnetic properties (e.g. signed/unsigned magnetic field strength) and area of the CH. As such, the evolutionary pattern in the magnetic field clearly follows the three-phase evolution (growing, maximum and decaying phase) as found from EUV data (Part I). This evolutionary process is most likely driven by strong FTs with a mean magnetic field strength exceeding 50 G. During the maximum phase they entail up to 72% of the total signed magnetic flux of the CH, but only cover up to 3.9% of the total CH area, whereas during the growing and decaying phase, strong FTs entail 54-60% of the signed magnetic flux and cover around 1-2% of the CHs total area. We conclude that small scale-structures of strong unipolar magnetic field are the fundamental building blocks of a CH and govern its evolution.

astro-ph.SR↗

arXiv2019

CME volume calculation from 3D GCS reconstruction

The mass evolution of a coronal mass ejection (CME) is an important parameter characterizing the drag force acting on a CME as it propagates through interplanetary space. Spacecraft measure in-situ plasma densities of CMEs during crossing events, but for investigating the mass evolution, we also need to know the CME geometry, more specific, its volume. Having derived the CME volume and mass from remote sensing data using 3D reconstructed CME geometry, we can calculate the CME density and compare it with in-situ proton density measurements near Earth. From that we may draw important conclusions on a possible mass increase as the CME interacts with the ambient solar wind in the heliosphere. In this paper we will describe in detail the method for deriving the CME volume using the graduated cylindrical shell (GCS) model (Thernisien et al., 2006,2009, see Figure 1). We show that, assuming self-similar expansion, one can derive the volume of the CME from two GCS parameters and that it furthermore can be expressed as a function of distance.

astro-ph.SR↗

arXiv2018

Observations of the solar chromosphere with ALMA and comparison with theoretical models

In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysis is to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere and corona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed. Based on a modified FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, the observational and modeling results are compared and discussed.

astro-ph.SR↗

arXiv2018

Benchmarking CME Arrival Time and Impact: Progress on Metadata, Metrics, and Events

Accurate forecasting of the arrival time and subsequent geomagnetic impacts of Coronal Mass Ejections (CMEs) at Earth is an important objective for space weather forecasting agencies. Recently, the CME Arrival and Impact working team has made significant progress towards defining community-agreed metrics and validation methods to assess the current state of CME modeling capabilities. This will allow the community to quantify our current capabilities and track progress in models over time. Firstly, it is crucial that the community focuses on the collection of the necessary metadata for transparency and reproducibility of results. Concerning CME arrival and impact we have identified 6 different metadata types: 3D CME measurement, model description, model input, CME (non-)arrival observation, model output data and metrics and validation methods. Secondly, the working team has also identified a validation time period, where all events within the following two periods will be considered: 1 January 2011-31 December 2012 and January 2015-31 December 2015. Those two periods amount to a total of about 100 hit events at Earth and a large amount of misses. Considering a time period will remove any bias in selecting events and the event set will represent a sample set that will not be biased by user selection. Lastly, we have defined the basic metrics and skill scores that the CME Arrival and Impact working team will focus on.

astro-ph.SR↗

arXiv2018

3-Phase Evolution of a Coronal Hole, Part I: 360° remote sensing and in-situ observations

We investigate the evolution of a well-observed, long-lived, low-latitude coronal hole (CH) over 10 solar rotations in the year 2012. By combining EUV imagery from STEREO-A/B and SDO we are able to track and study the entire evolution of the CH having a continuous 360$°$ coverage of the Sun. The remote sensing data are investigated together with in-situ solar wind plasma and magnetic field measurements from STEREO-A/B, ACE and WIND. From this we obtain how different evolutionary states of the CH as observed in the solar atmosphere (changes in EUV intensity and area) affect the properties of the associated high-speed stream measured at $1$AU. Most distinctly pronounced for the CH area, three development phases are derived: a) growing, b) maximum, and c) decaying phase. During these phases the CH area a) increases over a duration of around three months from about $1 \cdot 10^{10} \mathrm{km}^{2}$ to $6 \cdot 10^{10} \mathrm{km}^{2}$, b) keeps a rather constant area for about one month of $> 9 \cdot 10^{10} \mathrm{km}^{2}$, and c) finally decreases in the following three months below $1 \cdot 10^{10} \mathrm{km}^{2}$ until the CH cannot be identified anymore. The three phases manifest themselves also in the EUV intensity and in in-situ measured solar wind proton bulk velocity. Interestingly, the three phases are related to a different range in solar wind speed variations and we find for the growing phase a range of $460-600$~km~s$^{-1}$, for the maximum phase $600-720$~km~s$^{-1}$, and for the decaying phase a more irregular behavior connected to slow and fast solar wind speed of $350-550$~km~s$^{-1}$.

astro-ph.SR↗

arXiv2018

Coronal hole evolution from multi-viewpoint data as input for a STEREO solar wind speed persistence model

We present a concept study of a solar wind forecasting method for Earth, based on persistence modeling from STEREO in-situ measurements combined with multi-viewpoint EUV observational data. By comparing the fractional areas of coronal holes (CHs) extracted from EUV data of STEREO and SoHO/SDO, we perform an uncertainty assessment derived from changes in the CHs and apply those changes to the predicted solar wind speed profile at 1AU. We evaluate the method for the time period 2008-2012, and compare the results to a persistence model based on ACE in-situ measurements and to the STEREO persistence model without implementing the information on CH evolution. Compared to an ACE based persistence model, the performance of the STEREO persistence model which takes into account the evolution of CHs, is able to increase the number of correctly predicted high-speed streams by about 12%, and to decrease the number of missed streams by about 23%, and the number of false alarms by about 19%. However, the added information on CH evolution is not able to deliver more accurate speed values for the forecast than using the STEREO persistence model without CH information which performs better than an ACE based persistence model. Investigating the CH evolution between STEREO and Earth view for varying separation angles over ~25-140° East of Earth, we derive some relation between expanding CHs and increasing solar wind speed, but a less clear relation for decaying CHs and decreasing solar wind speed. This fact most likely prevents the method from making more precise forecasts. The obtained results support a future L5 mission and show the importance and valuable contribution using multi-viewpoint data.

astro-ph.SR↗

arXiv2017

Ensemble Prediction of a Halo Coronal Mass Ejection Using Heliospheric Imagers

The Solar TErrestrial RElations Observatory (STEREO) and its heliospheric imagers (HI) have provided us the possibility to enhance our understanding of the interplanetary propagation of coronal mass ejections (CMEs). HI-based methods are able to forecast arrival times and speeds at any target and use the advantage of tracing a CME's path of propagation up to 1 AU. In our study we use the ELEvoHI model for CME arrival prediction together with an ensemble approach to derive uncertainties in the modeled arrival time and impact speed. The CME from 3 November 2010 is analyzed by performing 339 model runs that are compared to in situ measurements from lined-up spacecraft MESSENGER and STEREO-B. Remote data from STEREO-B showed the CME as halo event, which is comparable to an HI observer situated at L1 and observing an Earth-directed CME. A promising and easy approach is found by using the frequency distributions of four ELEvoHI output parameters, drag parameter, background solar wind speed, initial distance and speed. In this case study, the most frequent values of these outputs lead to the predictions with the smallest errors. Restricting the ensemble to those runs, we are able to reduce the mean absolute arrival time error from $3.5 \pm 2.6$ h to $1.6 \pm 1.1$ h at 1 AU. Our study suggests that L1 may provide a sufficient vantage point for an Earth-directed CME, when observed by HI, and that ensemble modeling could be a feasible approach to use ELEvoHI operationally.

astro-ph.SR↗

arXiv2017

The causes of quasi-homologous CMEs

In this paper, we identified the magnetic source locations of 142 quasi- homologous (QH) coronal mass ejections (CMEs), of which 121 are from solar cycle (SC) 23 and 21 from SC 24. Among those CMEs, 63% originated from the same source location as their predecessor (defined as S-type), while 37% origi- nated from a different location within the same active region as their predecessor (defined as D-type). Their distinctly different waiting time distribution, peaking around 7.5 and 1.5 hours for S- and D-type CMEs, suggests that they might involve different physical mechanisms with different characteristic time scales. Through detailed analysis based on non-linear force free (NLFF) coronal mag- netic field modeling of two exemplary cases, we propose that the S-type QH CMES might involve a recurring energy release process from the same source location (by magnetic free energy replenishment), whereas the D-type QH CMEs can happen when a flux tube system disturbed by a nearby CME.

astro-ph.SR↗

arXiv2017

On flare-CME characteristics from Sun to Earth combining remote-sensing image data with in-situ measurements supported by modeling

We analyze the well observed flare-CME event from October 1, 2011 (SOL2011-10-01T09:18) covering the complete chain of action - from Sun to Earth - for a better understanding of the dynamic evolution of the CME and its embedded magnetic field. We study the solar surface and atmosphere associated with the flare-CME from SDO and ground-based instruments, and also track the CME signature off-limb from combined EUV and white-light data with STEREO. By applying 3D reconstruction techniques (GCS, total mass) to stereoscopic STEREO-SoHO coronagraph data, we track the temporal and spatial evolution of the CME in interplanetary space and derive its geometry and 3D-mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the MC from in-situ measurements (Wind). This is compared to nonlinear force-free (NLFF) model results as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are reduced in interplanetary space by ~50% and 75%, respectively, possibly indicating to an erosion process. A mass increase of 10% for the CME is observed over the distance range from ~4-20 Rs. The temporal evolution of the CME associated core dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME.

astro-ph.SR↗

arXiv2017

Characteristics of Low-Latitude Coronal Holes near the Maximum of Solar cycle 24

We investigate the statistics of 288 low-latitude coronal holes extracted from SDO/AIA-193 filtergrams over the time range 2011/01/01 to 2013/12/31. We analyse the distribution of characteristic coronal hole properties, such as the areas, mean AIA-193 intensities, and mean magnetic field densities, the local distribution of the SDO/AIA-193 intensity and the magnetic field within the coronal holes, and the distribution of magnetic flux tubes in coronal holes. We find that the mean magnetic field density of all coronal holes under study is 3.0 +- 1.6 G, and the percentage of unbalanced magnetic flux is 49 +- 16 %. The mean magnetic field density, the mean unsigned magnetic field density, and the percentage of unbalanced magnetic flux of coronal holes depend strongly pairwise on each other, with correlation coefficients cc > 0.92. Furthermore, we find that the unbalanced magnetic flux of the coronal holes is predominantly concentrated in magnetic flux tubes: 38 % (81 %) of the unbalanced magnetic flux of coronal holes arises from only 1 % (10 %) of the coronal hole area, clustered in magnetic flux tubes with field strengths > 50 G (10 G). The average magnetic field density and the unbalanced magnetic flux derived from the magnetic flux tubes correlate with the mean magnetic field density and the unbalanced magnetic flux of the overall coronal hole (cc > 0.93). These findings give evidence that the overall magnetic characteristics of coronal holes are governed by the characteristics of the magnetic flux tubes.

astro-ph.SR↗

arXiv2016

ALMA Observations of the Sun in Cycle 4 and Beyond

This document was created by the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) in preparation of the first regular observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which are anticipated to start in ALMA Cycle 4 in October 2016. The science cases presented here demonstrate that a large number of scientifically highly interesting observations could be made already with the still limited solar observing modes foreseen for Cycle 4 and that ALMA has the potential to make important contributions to answering long-standing scientific questions in solar physics. With the proposal deadline for ALMA Cycle 4 in April 2016 and the Commissioning and Science Verification campaign in December 2015 in sight, several of the SSALMON Expert Teams composed strategic documents in which they outlined potential solar observations that could be feasible given the anticipated technical capabilities in Cycle 4. These documents have been combined and supplemented with an analysis, resulting in recommendations for solar observing with ALMA in Cycle 4. In addition, the detailed science cases also demonstrate the scientific priorities of the solar physics community and which capabilities are wanted for the next observing cycles. The work on this White Paper effort was coordinated in close cooperation with the two international solar ALMA development studies led by T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be further updated until the beginning of Cycle 4 in October 2016. In particular, we plan to adjust the technical capabilities of the solar observing modes once finally decided and to further demonstrate the feasibility and scientific potential of the included science cases by means of numerical simulations of the solar atmosphere and corresponding simulated ALMA observations.

astro-ph.SR↗

arXiv2016

Chromospheric evaporation flows and density changes deduced from Hinode/EIS during an M1.6 flare

We analyzed high-cadence sit-and-stare observations acquired with the Hinode/EIS spectrometer and HXR measurements acquired with RHESSI during an M-class flare. During the flare impulsive phase, we observe no significant flows in the cooler Fe XIII line but strong upflows, up to 80-150 km/s, in the hotter Fe XVI line. The largest Doppler shifts observed in the Fe XVI line were co-temporal with the sharp intensity peak. The electron density obtained from a Fe XIII line pair ratio exhibited fast increase (within two minutes) from the pre-flare level of 5.01x10^(9) cm^(-3) to 3.16x10^(10) cm^(-3) during the flare peak. The nonthermal energy flux density deposited from the coronal acceleration site to the lower atmospheric layers during the flare peak was found to be 1.34x10^(10) erg/s/cm^(2) for a low-energy cut-off that was estimated to be 16 keV. During the decline flare phase, we found a secondary intensity and density peak of lower amplitude that was preceded by upflows of 15 km/s that were detected in both lines. The flare was also accompanied by a filament eruption that was partly captured by the EIS observations. We derived Doppler velocities of 250-300 km/s for the upflowing filament material.The spectroscopic results for the flare peak are consistent with the scenario of explosive chromospheric evaporation, although a comparatively low value of the nonthermal energy flux density was determined for this phase of the flare. This outcome is discussed in the context of recent hydrodynamic simulations. It provides observational evidence that the response of the atmospheric plasma strongly depends on the properties of the electron beams responsible for the heating, in particular the steepness of the energy distribution.

astro-ph.SR↗

arXiv2015

Thermosphere and geomagnetic response to interplanetary coronal mass ejections observed by ACE and GRACE: Statistical results

For the period July 2003 to August 2010, the interplanetary coronal mass ejection (ICME) catalogue maintained by Richardson and Cane lists 106 Earth-directed events, which have been measured in-situ by plasma and field instruments onboard the ACE satellite. We present a statistical investigation of the Earth's thermospheric neutral density response by means of accelerometer measurements collected by the GRACE satellites, which are available for 104 ICMEs in the data set, and its relation to various geomagnetic indices and characteristic ICME parameters such as the impact speed, southward magnetic field strength (Bz). The majority of ICMEs causes a distinct density enhancement in the thermosphere, with up to a factor of eight compared to the pre-event level. We find high correlations between ICME Bz and thermospheric density enhancements (~0.9), while the correlation with the ICME impact speed is somewhat smaller (~0.7). The geomagnetic indices revealing the highest correlations are Dst and SYM-H (~0.9), the lowest correlations are obtained for kp and AE (~0.7), which show a nonlinear relation with the thermospheric density enhancements. Separating the response for the shock sheath region and the magnetic structure of the ICME, we find that the Dst and SYM-H reveal a tighter relation to the Bz minimum in the magnetic structure of the ICME, whereas the polar cap indices show higher correlations with the Bz minimum in the shock sheath region. Since the strength of the Bz component - either in the sheath or the magnetic structure of the ICME - is highly correlated (~0.9) with the neutral density enhancement, we discuss the possibility of satellite orbital decay estimates based on magnetic field measurements at L1, i.e. before the ICME hits the Earth's magnetosphere. This will further stimulate progress in space weather understanding and applications regarding satellite operations.

astro-ph.EP↗

arXiv2015

Propagation of the 7 January 2014 CME and Resulting Geomagnetic Non-Event

On 7 January 2014 an X1.2 flare and CME with a radial speed $\approx$2500 km s$^{-1}$ was observed from near an active region close to disk center. This led many forecasters to estimate a rapid arrival at Earth ($\approx$36 hours) and predict a strong geomagnetic storm. However, only a glancing CME arrival was observed at Earth with a transit time of $\approx$49 hours and a $K_{\rm P}$ geomagnetic index of only $3-$. We study the interplanetary propagation of this CME using the ensemble Wang-Sheeley-Arge (WSA)-ENLIL+Cone model, that allows a sampling of CME parameter uncertainties. We explore a series of simulations to isolate the effects of the background solar wind solution, CME shape, tilt, location, size, and speed, and the results are compared with observed in-situ arrivals at Venus, Earth, and Mars. Our results show that a tilted ellipsoid CME shape improves the initial real-time prediction to better reflect the observed in-situ signatures and the geomagnetic storm strength. CME parameters from the Graduated Cylindrical Shell model used as input to WSA--ENLIL+Cone, along with a tilted ellipsoid cloud shape, improve the arrival-time error by 14.5, 18.7, 23.4 hours for Venus, Earth, and Mars respectively. These results highlight that CME orientation and directionality with respect to observatories play an important role in understanding the propagation of this CME, and for forecasting other glancing CME arrivals. This study also demonstrates the importance of three-dimensional CME fitting made possible by multiple viewpoint imaging.

astro-ph.SR↗

arXiv2015

Heliospheric Propagation of Coronal Mass Ejections: Drag-Based Model Fitting

The so-called drag-based model (DBM) simulates analytically the propagation of coronal mass ejections (CMEs) in interplanetary space and allows the prediction of their arrival times and impact speeds at any point in the heliosphere ("target"). The DBM is based on the assumption that beyond a distance of about 20 solar radii from the Sun, the dominant force acting on CMEs is the "aerodynamic" drag force. In the standard form of DBM, the user provisionally chooses values for the model input parameters, by which the kinematics of the CME over the entire Sun--"target" distance range is defined. The choice of model input parameters is usually based on several previously undertaken statistical studies. In other words, the model is used by ad hoc implementation of statistics-based values of the input parameters, which are not necessarily appropriate for the CME under study. Furthermore, such a procedure lacks quantitative information on how well the simulation reproduces the coronagraphically observed kinematics of the CME, and thus does not provide an estimate of the reliability of the arrival prediction. In this paper we advance the DBM by adopting it in a form that employs the CME observations over a given distance range to evaluate the most suitable model input parameters for a given CME by means of the least-squares fitting. Furthermore, the new version of the model automatically responds to any significant change of the conditions in the ambient medium (solar wind speed, density, CME--CME interactions, etc.) by changing the model input parameters according to changes in the CME kinematics. The advanced DBM is shaped in a form that can be readily employed in an operational system for real-time space-weather forecasting by promptly adjusting to a successively expanding observational dataset, thus providing a successively improving prediction of the CME arrival.

astro-ph.SR↗