arXiv subjects

Merav Opher

Publications and source records attributed to Merav Opher.

34 records · Page 2Linked to original sources

arXiv2018

Globally Distributed Energetic Neutral Atom Maps for the "Croissant" Heliosphere

A recent study by Opher et al. (2015) suggested the heliosphere has a "croissant" shape, where the heliosheath plasma is confined by the toroidal solar magnetic field. The "croissant" heliosphere is in contrast to the classically accepted view of a comet-like tail. We investigate the effect of the "croissant" heliosphere model on energetic neutral atom (ENA) maps. Regardless of the existence of a split tail, the confinement of the heliosheath plasma should appear in ENA maps. ENA maps from the Interstellar Boundary Explorer (IBEX) have shown two high latitude lobes with excess ENA flux at higher energies in the tail of the heliosphere. These lobes could be a signature of the confinement of the heliosheath plasma, while some have argued they are caused by the fast/slow solar wind profile. Here we present ENA maps of the "croissant" heliosphere, focusing on understanding the effect of the heliosheath plasma collimation by the solar magnetic field while using a uniform solar wind. We incorporate pick-up ions (PUIs) into our model based on Malama et al. (2006) and Zank et al. (2010). We use the neutral solution from our MHD model to determine the angular variation of the PUIs, and include the extinction of PUIs in the heliosheath. In the presence of a uniform solar wind, we find that the collimation in the "croissant" heliosphere does manifest itself into two high latitude lobes of increased ENA flux in the downwind direction.

astro-ph.SR↗

arXiv2015

Voyager 2 solar plasma and magnetic field spectral analysis for intermediate data sparsity

The Voyager probes are the furthest, still active, spacecraft ever launched from Earth. During their 38-year trip, they have collected data regarding solar wind properties (such as the plasma velocity and magnetic field intensity). Unfortunately, a complete time evolution of the measured physical quantities is not available. The time series contains many gaps which increase in frequency and duration at larger distances. The aim of this work is to perform a spectral and statistical analysis of the solar wind plasma velocity and magnetic field using Voyager 2 data measured in 1979, when the gaps/signal ratio is of order of unity. This analysis is achieved using four different data reconstruction techniques: averages on linearly interpolated subsets, correlation of linearly interpolated data, compressed sensing spectral estimation, and maximum likelihood data reconstruction. With five frequency decades, the spectra we obtained have the largest frequency range ever computed at 5 astronomical units from the Sun; spectral exponents have been determined for all the components of the velocity and magnetic field fluctuations. Void analysis is also useful in recovering other spectral properties such as integral scales (see for instance Table 4) and, if the confidence level of the measurements is sufficiently high, the decay variation in the small scale range due, for instance, to dissipative effects.

physics.space-ph↗

arXiv2014

Global Numerical Modeling of Energetic Proton Acceleration in a Coronal Mass Ejection Traveling through the Solar Corona

The acceleration of protons and electrons to high (sometimes GeV/nucleon) energies by solar phenomena is a key component of space weather. These solar energetic particle (SEP) events can damage spacecraft and communications, as well as present radiation hazards to humans. In-depth particle acceleration simulations have been performed for idealized magnetic fields for diffusive acceleration and particle propagation, and at the same time the quality of MHD simulations of coronal mass ejections (CMEs) has improved significantly. However, to date these two pieces of the same puzzle have remained largely decoupled. Such structures may contain not just a shock but also sizable sheath and pileup compression regions behind it, and may vary considerably with longitude and latitude based on the underlying coronal conditions. In this work, we have coupled results from a detailed global three-dimensional MHD time-dependent CME simulation to a global proton acceleration and transport model, in order to study time-dependent effects of SEP acceleration between 1.8 and 8 solar radii in the 2005 May 13 CME. We find that the source population is accelerated to at least 100 MeV, with distributions enhanced up to six orders of magnitude. Acceleration efficiency varies strongly along field lines probing different regions of the dynamically evolving CME, whose dynamics is influenced by the large-scale coronal magnetic field structure. We observe strong acceleration in sheath regions immediately behind the shock.

physics.space-ph↗

arXiv2013

On the Rotation of the Magnetic Field Across the Heliopause

There was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). This rotation was used as one of the criteria to determine if Voyager 1 (V1) had crossed the HP. Recently the Voyager team concluded that V1 crossed into interstellar space last year (Gurnett et al. 2013). The question is then why there was no significant rotation in the direction of the magnetic field across the HP (Burlaga et al. 2013). Here we present simulations that reveal that strong rotations in the direction of the magnetic field at the HP at the location of V1 (and Voyager 2) are not expected. The solar magnetic field strongly affects the drapping of the interstellar magnetic field (BISM) around the HP. BISM twists as it approaches the HP and acquires a strong T component (east-west). The strong increase in the T component occurs where the interstellar flow stagnates in front of the HP. At this same location the N component BN is significantly reduced. Above and below the neighboring interstellar magnetic field lines also twist into the T direction. This behavior occurs for a wide range of orientations of BISM. Thus, the BN component at the V1 location and the associated angle delta=a sin(BN/B) are small (around 10-20 degrees), as seen in the observations (Burlaga et al. 2013). Only after some significant distance outside the HP, is the direction of the interstellar field distinguishably different from that of the Parker spiral.

astro-ph.SR↗

arXiv2013

Forecasting a Coronal Mass Ejection's Altered Trajectory: ForeCAT

To predict whether a coronal mass ejection (CME) will impact Earth, the effects of the background on the CME's trajectory must be taken into account. We develop a model, ForeCAT (Forecasting a CME's Altered Trajectory), of CME deflection due to magnetic forces. ForeCAT includes CME expansion, a three-part propagation model, and the effects of drag on the CME's deflection. Given the background solar wind conditions, the launch site of the CME, and the properties of the CME (mass, final propagation speed, initial radius, and initial magnetic strength), ForeCAT predicts the deflection of the CME. Two different magnetic backgrounds are considered: a scaled background based on type II radio burst profiles and a Potential Field Source Surface (PFSS) background. For a scaled background where the CME is launched from an active region located between a CH and streamer region the strong magnetic gradients cause a deflection of 8.1 degrees in latitude and 26.4 degrees in longitude for a 1e15 g CME propagating out to 1 AU. Using the PFSS background, which captures the variation of the streamer belt position with height, leads to a deflection of 1.6 degrees in latitude and 4.1 degrees in longitude for the control case. Varying the CME's input parameters within observed ranges leads to the majority of CMEs reaching the streamer belt within the first few solar radii. For these specific backgrounds, the streamer belt acts like a potential well that forces the CME into an equilibrium angular position.

astro-ph.SR↗

arXiv2012

Multi-ion, multi-fluid 3-D magnetohydrodynamic simulation of the outer heliosphere

Data from the Voyager probes and the Interstellar Boundary Explorer have revealed the importance of pick-up ions (PUIs) in understanding the character and behavior of the outer heliosphere, the region of interaction between the solar wind and the interstellar medium. In the outer heliosphere PUIs carry a large fraction of the thermal pressure, which effects the nature of the termination shock, and they are a dominate component of pressure in the heliosheath. This paper describes the development of a new multi-ion, multi-fluid 3-D magnetohydrodynamic model of the outer heliosphere. This model has the added capability of tracking the individual fluid properties of multiple ion populations. For this initial study two ion populations are modeled: the thermal solar wind ions and PUIs produced in the supersonic solar wind. The model also includes 4 neutral fluids that interact through charge-exchange with the ion fluids. The new multi-ion simulation reproduces the significant heating of PUIs at the termination shock, as inferred from Voyager observations, and provides properties of PUIs in the 3-D heliosheath. The thinning of the heliosheath due to the loss of thermal energy in the heliosheath from PUI and neutral interaction is also quantified. In future work the multi-ion, multi-fluid model will be used to simulate energetic neutral atom (ENA) maps for comparison with the Interstellar Boundary Explorer, particularly at PUI energies of less than 1 keV.

astro-ph.SR↗

arXiv2011

Kinetic vs. multi-fluid approach for interstellar neutrals in the heliosphere: exploration of the interstellar magnetic field effects

We present a new 3d self-consistent two-component (plasma and neutral hydrogen) model of the solar wind interaction with the local interstellar medium (LISM). This model (K-MHD) combines the MHD treatment of the solar wind and the ionized LISM component, with a kinetic model of neutral interstellar hydrogen (LISH). The local interstellar magnetic field (BLISM) intensity and orientation are chosen based on an early analysis of the heliosheath flows (Opher et al. 2009). The properties of the plasma and neutrals obtained using the (K-MHD) model are compared to previous multi-fluid (Opher et al. 2009) and kinetic models (Izmodenov et al. 2005). The new treatment of LISH revealed important changes in the heliospheric properties not captures by the multi-fluid model. These include a decrease in the heliocentric distance to the termination shock (TS), a thinner heliosheath and a reduced deflection angle (θ) of the heliosheath flows. The asymmetry of the termination shock, however, seems to be unchanged by the kinetic aspect of the LISH.

astro-ph.SR↗

arXiv2008

Three-Dimensional MHD Simulation of the 2003 October 28 Coronal Mass Ejection: Comparison with LASCO Coronagraph Observations

We numerically model the coronal mass ejection (CME) event of October 28, 2003 that erupted from active region 10486 and propagated to Earth in less than 20 hours causing severe geomagnetic storms. The magnetohydrodynamic (MHD) model is formulated by first arriving at a steady state corona and solar wind employing synoptic magnetograms. We initiate two CMEs from the same active region, one approximately a day earlier that preconditions the solar wind for the much faster CME on the 28th. This second CME travels through the corona at a rate of over 2500 km s$^{-1}$ driving a strong forward shock. We clearly identify this shock in an image produced by the Large Angle Spectrometric Coronagraph (LASCO) C3, and reproduce the shock and its appearance in synthetic white light images from the simulation. We find excellent agreement with both the general morphology and the quantitative brightness of the model CME with LASCO observations. These results demonstrate that the CME shape is largely determined by its interaction with the ambient solar wind and may not be sensitive to the initiation process. We then show how the CME would appear as observed by wide-angle coronagraphs onboard the Solar Terrestrial Relations Observatory (STEREO) spacecraft. We find complex time evolution of the white-light images as a result of the way in which the density structures pass through the Thomson sphere. The simulation is performed with the Space Weather Modeling Framework (SWMF).

astro-ph↗

arXiv2006

Global Asymmetry of the Heliosphere

Opher et al. 2006 showed that an interstellar magnetic field parallel to the plane defined by the deflection of interstellar hydrogen atoms can produce a north/south asymmetry in the distortion of the solar wind termination shock. This distortion is consistent with Voyager 1 and Voyager 2 observations of the direction of field-aligned streaming of the termination shock particles upstream the shock. The model also indicates that such a distortion will result in a significant north/south asymmetry in the distance to the shock and the thickness of heliosheath. The two Voyager spacecraft should reveal the nature and degree of the asymmetry in the termination shock and heliosheath.

astro-ph↗

arXiv2006

Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations

We show that that an interstellar magnetic field can produce a north/south asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements, we suggest that the angle $α$ between the interstellar wind velocity and magnetic field is $30^{\circ} < α< 60^{\circ}$. The distortion of the shock is such that termination shock particles could stream outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within $\sim 2~AU$ of the shock. The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within $\sim 5~AU$ of the shock, but with particles from the shock streaming inward along the field. Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years.

astro-ph↗

arXiv2001

Nuclear reaction rates and energy in stellar plasmas : The effect of highly damped modes

The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wavenumbers $k \gg k_{D}$, even being only weakly excited, with less than $k_{B}T$ per mode, make a significant contribution to the energy and screening in a plasma. When the de Broglie wavelength is much less than the distance of closest approach of thermal electrons, a classical analysis of the plasma can

astro-ph↗

arXiv2000

Nuclear Reaction Rates in a Plasma: The Effect of Highly Damped Modes

The fluctuation-dissipation theorem is used to evaluate the screening factor of nuclear reactions due to the electromagnetic fluctuations in a plasma. We show that the commonly used Saltpeter factor is obtained if only fluctuations near the plasma eigenfrequency are assumed to be important (ω\sim ω_{pe}\ll T (\hbar=k_{B}=1)). By taking into account all the fluctuations, the highly damped ones, with ω>ω_{pe}, as well as those with ω\leqω_{pe}, we find that nuclear reaction rates are higher than those obtained using the Saltpeter factor, for many interesting plasmas.

astro-ph↗

arXiv1999

Dynamic Screening in Thermonuclear Reactions

It has recently been argued that there are no dynamic screening corrections to Salpeter's enhancement factor in thermonuclear reactions, in the weak-screening limit. The arguments used were: 1) The Gibbs probability distribution is factorable into two parts, one of which, $exp(-β\sum e_{i}e_{j}/r_{ij})$ ($β=1/k_{B}T$), is independent of velocity space; and 2) The enhancement factor is $w=1+β^{2}e^{2}Z_{1}Z_{2}< ϕ^{2} >$ with ${< ϕ^{2} >}_{k}={< E^{2} >}_{k}/k^{2}$ and ${< E^{2} >}_{k}/(8π)=(T/2)[1-ε^{-1} (0,k)]$. We show that both of these arguments are incorrect.

astro-ph↗

arXiv1999

The Energy of a Plasma in the Classical Limit

When λ_{T} << d_{T}, where λ_{T} is the de Broglie wavelength and d_{T}, the distance of closest approach of thermal electrons, a classical analysis of the energy of a plasma can be made. In all the classical analysis made until now, it was assumed that the frequency of the fluctuations ω<< T (k_{B}=\hbar=1). Using the fluctuation-dissipation theorem, we evaluate the energy of a plasma, allowing the frequency of the fluctuations to be arbitrary. We find that the energy density is appreciably larger than previously thought for many interesting plasmas, such as the plasma of the Universe before the recombination era.

astro-ph↗

arXiv1997

The Magnetic Field Spectrum in a Plasma in Thermal Equilibrium in the Epoch of Primordial Nucleosynthesis

The low frequency magnetic field spectrum in the primordial plasma is of particular interest as a possible origin of magnetic fields in the universe (e.g., Tajima et al. 1992 and Cable and Tajima 1992). We derive the magnetic field spectrum in the primordial plasma, in particular, at the epoch of primordial nucleosynthesis. The pioneering study of Cable and Tajima (1992) of the electromagnetic fluctuations, based on the Fluctuation-Dissipation Theorem, is extended. Our model describes both the thermal and collisional effects in a plasma. It is based on a kinetic description with the BGK collision term. It is shown that the zero-frequency peak found by Cable and Tajima (1992) decreases. At high frequencies, the blackbody spectrum is obtained naturally without the necessity of the link procedure used by them. At low frequencies ($ω\leq 4ω_{pe}$, where $ω_{pe}$ is the electron plasma frequency) it is shown that the magnetic field spectrum has more energy than the blackbody spectrum in vacuum.

astro-ph↗

arXiv1997

Was The Electromagnetic Spectrum A Blackbody Spectrum In The Early Universe?

It is assumed, in general, that the electromagnetic spectrum in the Primordial Universe was a blackbody spectrum in vacuum. We derive the electromagnetic spectrum, based on the Fluctuation-Dissipation Theorem that describes the electromagnetic fluctuations in a plasma. Our description includes thermal and collisional effects in a plasma. The electromagnetic spectrum obtained differs from the blackbody spectrum in vacuum at low frequencies. In particular, concentrating on the primordial nucleosynthesis era, it has more energy for frequencies less than 3 to $6ω_{pe}$, where $ω_{pe}$ is the electron plasma frequency.

astro-ph↗