Accelerated magnetosheath flows caused by IMF draping: Dependence on latitude

Aims. We study how a fast solar wind stream embedded in a slow solar wind influences the spread of solar energetic protons in interplanetary space. In particular, we aim at understanding how the particle intensity and anisotropy vary along interplanetary magnetic field (IMF) lines that encounter changing solar wind conditions such as the shock waves bounding a corotating interaction region (CIR). Moreover, we study how the intensities and anisotropies vary as a function of the longitudinal and latitudinal coordinate, and how the width of the particle intensities evolves with the heliographic radial distance. Furthermore, we study how cross-field diffusion may alter these spatial profiles. Methods. To model the energetic protons, we used a recently developed particle transport code that computes particle distributions in the heliosphere by solving the focused transport equation (FTE) in a stochastic manner. The particles are propagated in a solar wind containing a CIR, which was generated by the heliospheric model, EUHFORIA. We study four cases in which we assume a delta injection of 4 MeV protons spread uniformly over different regions at the inner boundary of the model. These source regions have the same size and shape, yet are shifted in longitude from each other, and are therefore magnetically connected to different solar wind conditions. Results. The intensity and anisotropy profiles along selected IMF lines vary strongly according to the different solar wind conditions encountered along the field line. The IMF lines crossing the shocks bounding the CIR show the formation of accelerated particle populations, with the reverse shock wave being a more efficient accelerator than the forward shock wave. The longitudinal intensity profiles near the CIR are highly asymmetric in contrast to the profiles obtained in a nominal solar wind. For the injection regions that do not cross the transition zone between the fast and slow solar wind, we observe a steep intensity drop of several orders of magnitude near the stream interface (SI) inside the CIR. Moreover, we demonstrate that the longitudinal width of the particle intensity distribution can increase, decrease, or remain constant with heliographic radial distance, reflecting the underlying IMF structure. Finally, we show how the deflection of the IMF at the shock waves and the compression of the IMF in the CIR deforms the three-dimensional shape of the particle distribution in such a way that the original shape of the injection profile is lost.

The magnetospheric cusps are focal points in studies of both magnetic reconnection at the magnetopause and plasma entry into the magnetosphere. Contrary to a well‐understood precipitating thermal magnetosheath ion population, the origins of energetic ions in the cusp regions are still a matter of controversy. It has been suggested that these cusp energetic particles (CEP) with significant fluxes from magnetosheath energies up to several hundred keV/e are accelerated locally in the cusp. A recent paper has suggested local plasma conditions conducive to CEP acceleration in the cusp diamagnetic cavity (CDC). An alternative source region for CEP events is the quasi‐parallel bow shock, which is a well‐known particle accelerator. Energetic ions accelerated at the bow shock can be transported downstream and enter the cusp along newly reconnected field lines. Composition and energy spectra of these CEP events resemble those of bow shock energetic diffuse ions. We use recently developed techniques to determine the location of the reconnection site at the magnetopause, draping interplanetary magnetic field (IMF) lines over the magnetopause and mapping those field lines back into the solar wind to show the magnetic connection between the cusp regions, the Earth bow shock, and the upstream region. Several cusp crossings by the Polar satellite during variable IMF conditions are analyzed for patterns between the cusp, their connection to the upstream region, and the appearance of energetic ions in the cusp. Local plasma conditions in the cusp are also documented. This analysis reveals that the occurrence of CEP events is not uniquely determined by local plasma conditions. The flux of CEP ions depends on the location of the quasi‐parallel bow shock and the magnetic topology in the magnetosheath. Our analysis allows us to use CEP ions as tracers for plasma transport into the cusp and to better understand the magnetic topology between the solar wind and the ionosphere.

This study examines how solar wind plasma enters the magnetosphere under northward interplanetary magnetic field (IMF) conditions, using the Open Geospace General Circulation Model (OpenGGCM) for various solar wind, IMF, and geomagnetic dipole conditions. We trace flow paths of individual fluid elements from the solar wind and study the variation of the topology of the magnetic field line along those flow paths. We find that there is an entry window through which the solar wind plasma can enter the magnetosphere as a result of double high‐latitude reconnection under northward IMF conditions. We investigate how the entry window depends on solar wind, IMF, and geomagnetic dipole parameters, and we estimate the solar wind plasma entry rate for various conditions. We find that the effective entry rate under northward IMF conditions is of the order of 1026 to 1027 particles per second. We also estimate the conditions for which solar wind plasma entry is most efficient. The newly created flux tubes with closed‐field topology are subsequently convected to the nightside and consequently cause magnetosheath plasma to be captured and enter the magnetosphere. Some captured dayside plasma takes about 90 min to convect along the magnetopause to a near tail flank region of the central plasma sheet, thus forming a cold dense plasma sheet. Double high‐latitude reconnection can also release the captured plasma. Thus a balance of inflow and outflow of the captured plasma is eventually established under prolonged northward IMF conditions. We find that high‐latitude reconnection is common under northward IMF conditions in our simulations. It occurs for IMF with any clock angle within [−90°, 90°], measured in front of the bow shock, and for any geomagnetic dipole tilt angle within [−30°, 30°]. An IMF field line with a zero x component usually first reconnects with a geomagnetic field line at the northern high‐latitude boundary when the geomagnetic dipole tilts positive toward the Sun, and vice versa for negative dipole tilt. Our simulations also show that ionosphere conductance affects the convection of the newly closed field lines. Consequently, ionosphere conductance can change the solar wind plasma effective entry rate and can cause asymmetric effective entry rate with respect to positive and negative IMF clock angles.

We analyze the relativistic proton emission from the Sun during the eruptive event on 2017 September 10, which caused a ground-level enhancement (GLE 72) registered by the worldwide network of neutron monitors. Using the neutron monitor data and interplanetary transport modeling both along and across interplanetary magnetic field (IMF) lines, we deduce parameters of the proton injection into the interplanetary medium. The inferred injection profile of the interplanetary protons is compared with the profile of the >100 MeV γ-ray emission observed by the Fermi Large Area Telescope, attributed to pion production from the interaction of >300 MeV protons at the Sun. GLE 72 started with a prompt component that arrived along the IMF lines. This was followed by a more prolonged enhancement caused by protons arriving at the Earth across the IMF lines from the southwest. The interplanetary proton event is modeled using two sources—one source at the root of the Earth-connected IMF line and another source situated near the solar western limb. The maximum phase of the second injection of interplanetary protons coincides with the maximum phase of the prolonged >100 MeV γ-ray emission that originated from a small area at the solar western limb, below the current sheet trailing the associated coronal mass ejection (CME). A possible common source of interacting protons and interplanetary protons is discussed in terms of proton acceleration at the CME bow shock versus coronal (re-)acceleration in the wake of the CME.

Criticism of reconnection models of the magnetosphere

<p>The localized crustal magnetic anomalies scattered across the surface Mars create a unique, hybrid magnetosphere. Over regions of strong crustal magnetism (100-1000 nT at the surface), the crustal anomalies protrude up to thousands of kilometers in space and a create a “mini-magnetospheric” interaction with the draped interplanetary magnetic field (IMF). One consequence of this interaction is the occurrence of discrete aurora on the nightside of Mars via electron precipitation and energy deposition into the planet’s atmosphere stimulating light emission (<em>Bertaux et al.,</em> 2005). Discrete auroral emissions from electron impact with atmospheric CO<sub>2</sub> in the mid ultra-violet have been observed via the Imaging Ultra-Violet Spectrograph (IUVS) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. These emissions have been preferentially observed over the region of strong crustal magnetism in the southern hemisphere (~153°-213° E longitude, 30°S-70°S latitude) and favor “open” magnetic field lines (i.e., one end connected to the IMF, the other connected to the planet) (<em>Schneider et al., </em>2021). These trends suggest discrete aurora at Mars may form via magnetic reconnection between the strong crustal anomalies and the draped IMF around the planet. Magnetic reconnection is a process that can “open-up” the closed crustal magnetic fields at Mars and accelerate electrons along these magnetic field lines. However, there has yet to be a study that links these two phenomena directly, and as a result, our understanding of discrete auroral formation at Mars remains poorly understood.</p> <p><img src="" alt="" /></p> <p>In this study, we uncover the relationship between magnetic reconnection and discrete aurora at Mars by analyzing the draped magnetic field conditions during 78 discrete auroral emissions observed via nadir observations taken by the IUVS within the region of strong crustal fields. First, we simplify the crustal field region into its two primary constituents: one crustal anomaly that is positioned in the northern half of the region and closes northward, and one crustal anomaly that is positioned in the southern half of the region and closes southward (Figure <strong>1</strong>). We note that the discrete auroral emissions tend to fall on the “footprints” of the two crustal anomalies where the dominant component of the crustal magnetic field points radially into or out of the planet. Two plasma regimes are more susceptible to reconnection when their magnetic field orientations are anti-parallel to one another. Therefore, we hypothesize that if crustal field-draped IMF magnetic reconnection plays a role in discrete auroral formation over the strong crustal anomalies at Mars, then a southward draped IMF would activate auroral emissions over the northern crustal anomaly and a northward draped IMF would activate auroral emissions over the southern crustal anomaly.</p> <p><img src="" alt="" /></p> <p>We estimate the orientation of the draped magnetic field over this region of strong crustal magnetism using data obtained from the magnetometer instrument onboard MAVEN along the same orbit in a discrete auroral emission was detected by IUVS. We find that the estimated orientation of the draped magnetic field associated with each discrete auroral event shows a regional dependence consistent with our hypothesis: discrete auroral emissions over the footprints of the northern anomaly are associated with southward draped fields, and emissions over the footprints of the southern anomaly are associated with northward draped fields (Figure <strong>2</strong>). We also demonstrate that these findings hold implications for other trends reported in previous studies of discrete aurora over this region of crustal fields, namely a trend in local time and upstream IMF conditions. This study is the first to more definitively link magnetic reconnection to discrete aurora formation on the nightside of Mars.</p> <p>Bertaux, J., F. Leblanc, O. Witasse, E. Quemerais, J. Lilensten, S. Stern, B. Sandel, and O. Korablev (2005), Discovery of an aurora on Mars, Nature, 435(7043), 790-794.</p> <p>Schneider, N., et al. (2021), Discrete Aurora on Mars: Insights Into Their Distribution and Activity From MAVEN/IUVS Observations, Journal of Geophysical Research-Space Physics, 126(10).</p>

In this paper we discuss configurations of the heliospheric interface shaped by the influence of both the uniform interstellar and the interplanetary (Parker model — Archimedean spiral) magnetic fields in the presence of the neutral particles. Magnetic fields strongly affect the structure and the shape of the heliospheric interface, which becomes asymmetric for any configuration of the interstellar magnetic field (ISMF), for the case where the interplanetary magnetic field (IMF) is included. In particular, the interstellar magnetic field lines may merge with interplanetary magnetic field lines of opposite direction at the heliopause. This may possibly cause a reconnection of the magnetic field lines at the heliopause.

A model of partially open magnetosphere

A three‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation was used to model the interaction between the solar wind and the plasmas produced from cometary gases. The model, which includes cometary mass loading, reproduced several features observed by the recent missions to comet Halley: (1) a weak bow shock was formed 3.2×105 km in front of the comet, (2) the magnetic field increased by a factor of 3.7 across this weak bow shock and continued to increase up to the contact surface (Bmax/BIMF = ∼6 ‐ 8), and (3) the plasma temperature increased across the bow shock but decreased nearer to the comet. In addition, interplanetary magnetic field (IMF) lines were hung up on the comet and formed a long tail in which the lobe magnetic field was quite strong (Blobe/BIMF ∼ 5). A cold dense plasma sheet oriented normal to the IMF direction formed in the tail.

The resonance regions for resonant interactions of radiation belt electrons with obliquely propagating whistler-mode chorus waves are investigated in detail in the Dungey magnetic fields that are parameterized by the intensity of uniform southward interplanetary magnetic field (IMF) Bz or, equivalently, by the values of D=(M/Bz,0)1/3 (where M is the magnetic moment of the dipole and Bz,0 is the uniform southward IMF normal to the dipole’s equatorial plane). Adoption of background magnetic field model can considerably modify the determination of resonance regions. Compared to the results for the case of D = 50 (very close to the dipole field), the latitudinal coverage of resonance regions for 200 keV electrons interacting with chorus waves tends to become narrower for smaller D-values, regardless of equatorial pitch angle, resonance harmonics, and wave normal angle. In contrast, resonance regions for 1 MeV electrons tend to have very similar spatial lengths along the field line for various Dungey magnetic field models but cover different magnetic field intervals, indicative of a strong dependence on electron energy. For any given magnetic field line, the resonance regions where chorus-electron resonant interactions can take place rely closely on equatorial pitch angle, resonance harmonics, and kinetic energy. The resonance regions tend to cover broader latitudinal ranges for smaller equatorial pitch angles, higher resonance harmonics, and lower electron energies, consistent with the results in Ni and Summers [Phys. Plasmas 17, 042902, 042903 (2010)]. Calculations of quasi-linear bounce-averaged diffusion coefficients for radiation belt electrons due to nightside chorus waves indicate that the resultant scattering rates differ from using different Dungey magnetic field models, demonstrating a strong dependence of wave-induced electron scattering effect on the adoption of magnetic field model. Our results suggest that resonant wave-particle interaction processes should be implemented into a sophisticated, accurate global magnetic field model to pursue comprehensive and complete models of radiation belt electron dynamics.

Spherical harmonic representations of the geomagnetic field based on ground-based and low-altitude spacecraft measurements adequately describe the field within several Earth radii of the Earth’s surface. As the internal field decreases with increasing distance from the Earth, external field sources become relatively more important. In the region 3–6 R E magnetospheric plasma inflates the field lines and decreases the field strength by an amount which is typically 10’s of gammas and occasionally 100’s of gammas. At greater distances on the day side of the Earth the solar wind compresses the field and produces equatorial field strengths of approximately 60 γ at 10 R E . Field lines near the magnetopause intersect the Earth at approximately 78° latitude in the sunward hemisphere. The solar wind drags high latitude field lines away from the Earth in the night hemisphere forming the geomagnetic tail and neutral sheet. The average equatorial field at 10 R E near midnight is approximately 8 γ and the best estimate for the last closed line of force near midnight in an average magnetosphere is 69°. Magnetic tail field lines away from the neutral sheet diverge in both the east-west and north-south directions and the average quiet time magnitude decreases from 16 γ at 20 R E to 7 γ at 80 R E . Within 6 R E of the equatorial plane the field strength is depressed by several gammas. Asymmetric field inflation in the magnetosphere occurs during magnetic disturbances with the largest effects concentrated in the evening quadrant. The tail configuration can deviate substantially from the average configuration during magnetic disturbances. Prior to a substorm the configuration is characterized by a maximum number of lines extending far into the tail and relatively little flux crossing the equatorial plane. After a substorm or during quiet times the tail is characterized by an increased number of field lines crossing the equatorial plane in cislunar space.

We have examined the dayside high‐latitude convection response to a sudden southward turning of the Interplanetary Magnetic Field (IMF) and found that the response is nearly instantaneous (<2 min) over a spatial region extending from ∼75° to 85° and from ∼9 to 16 MLT. Observations of the magnetic field were made with the WIND spacecraft in the solar wind and the GEOTAIL and IMP8 spacecraft in the magnetosheath. In the high‐latitude ionosphere, the HF radars of the Super Dual Auroral Radar Network (SuperDARN) were used to monitor the convection. Based on the magnetosheath flow of the gas dynamic approximation, the field lines of the new IMF state were draped over a large portion of the dayside magnetopause when the first significant indication of convection response was measured in the ionosphere. Significant magnetic field line draping accompanied by extended reconnection on the dayside magnetopause may help explain the rapid, large‐scale response of ionospheric convection.

A new method for simulating the three-dimensional dynamics of charged energetic particles in very broadband noncompressive magnetic turbulence is introduced. All scales within the primary inertial range of the turbulence observed in the solar wind near 1 AU are now included for the independent computations of both the particle dynamics and the turbulent magnetic field lines (MFLs). While previous theories of resonant particle pitch-angle (PA) scattering and transport in interplanetary magnetic fields had favored interpreting the observed depletions in the electron PA distributions (PADs) around 90° PA as evidence of poor scattering at low PA cosines, the computed particle dynamics reveal a very different reality. The MFL directions now vary on many scales, and the PADs are depleted around 90° PA due to nonresonant filtering of the particles that propagate at too large an angle to the local magnetic field. Rather than being too weak, the scattering through 90° PA is actually so strong that the particles (electrons and protons/ions) are reflected and trapped in the turbulent magnetic fields. While the low-frequency nonresonant turbulence produces ubiquitous magnetic traps that only let through particles with the most field-aligned velocities, higher-frequency near-gyroscale turbulence, when present, enhances particle transport by allowing the particles to navigate between magnetic traps. Finally, visualizing both particle trajectories and MFLs in the very same turbulence reveals a powerful tool for understanding the effects of the turbulent fields on the particle dynamics and cross-field transport. Some cross-field-line scattering, strongly amplified by MFL dispersal, results in a strong cross-field scattering of the particles. From this visualization, it also appears that near-gyroscale turbulence, previously known as gyroresonant turbulence, does not resonantly interact with the particles. The interaction between particles and fields at or near the gyroscale, though potentially strong, does not actually involve the periodic driving of a true resonance.

When the interplanetary magnetic field (IMF) is southward, most of the ionospheric potential is generated by merging between the IMF and the magnetospheric field. Typically, the ionospheric potential responds linearly to the magnitude of the southward IMF. However, when the IMF magnitude is large, the ionospheric potential saturates and it becomes relatively insensitive to further increases in the IMF magnitude. We present evidence from simulations that under purely southward IMF conditions, the value of the portion of the potential due to reconnection is controlled by the divergence of the magnetosheath flow, which determines the geoeffective length in the solar wind. Typically, the gradient in the plasma pressure controls the magnetosheath flow, so as the southward IMF increases in magnitude, the change in the magnetosheath force balance is negligible, the geoeffective length in the solar wind does not change, and the reconnection potential increases linearly with the magnitude of the IMF. However, when the IMF magnitude increases to the point where J × B becomes the dominant force in the magnetosheath, further increases in IMF magnitude do affect the overall force balance, diverting more flow away from the merging line, decreasing the geoeffective length, and limiting the global merging rate. Thus magnetosheath force balance can be seen as a single organizing factor that regulates the geoeffective length in the solar wind for the entire range of solar wind parameters.

Charged energetic particles propagating in solar wind magnetic fields span field irregularities down to very short turbulent scales, not described by the original quasi-linear theory for weak magnetic turbulence. This theory only predicts a field line diffusion on the largest scales, well above the correlation length, inverse of the spectral flattening wavenumber. The quasi-linear prediction for the transport and behavior of magnetic field lines is generalized here to all scales and arbitrary three-dimensional turbulence spectra. New analytical expressions are derived for the field line mean square cross-field displacement ⟨Δx2⟩, and analytical proof is presented for the anomalous transport of the field lines. We find ⟨Δx2⟩ ∝ (Δz)β, where Δz is the elapsed distance along the average field and β, the transport exponent, can take any value between 0 and 2. A decreasing turbulence spectrum results in a field line supradiffusion (β > 1), while an inverted spectrum implies a subdiffusion (β < 1). Simple expressions are derived for the transport exponent and coefficient. A powerful new method is presented to compute magnetic field lines in the quasi-linear regime of turbulence that allows rapid computation of field lines generated from any three-dimensional turbulence spectrum, including some 1015 modes and more. Individual field lines computed with this method show how a spectral steepening results in a smoothing of the field lines and how harder spectra give increasingly more short-scale fluctuations. The field line self-similarity, characteristic of power-law spectra, is demonstrated visually, and the anomalous transport of the field lines is confirmed numerically.