P. H. Reiff

Empirical polar cap potentials

DMSP satellite plasma flow data from 1987–1990 are used to derive empirical models of the polar cap potential for quasi-steady interplanetary magnetic field (IMF) conditions. The large data set, due to the high duty cycle and nearly Sun synchronous DMSP orbits, allowed very stringent data selection criteria. The analysis indicates that a good description of the unskewed (Heppner Maynard pattern A) steady state polar cap potential is ΦA = 10−4v2+11.7B sin3 (θ/2) kV, where v is the solar wind velocity in kilometers per second, B is the magnitude of the interplanetary magnetic field in nanoteslas, and θ = arccos (Bz/|B|)GSM. The IMF-dependent contribution to the cross polar cap potential does not depend significantly on solar wind pressure. Functional forms for the potential do benefit from inclusion of an IMF independent term proportional to the solar wind flow energy. Best fits to IMF-independent contributions to the steady state polar cap potential yield ∼16 kV for vsw = 400 kilometers per second. During steady IMF the total unskewed polar cap potential drop is shown to be approximately ΦA = 16.5 + 15.5 Kp kV. The distribution of potential around the polar cap is examined as a function of magnetic local time. A sinusoidal distribution is an excellent description of the distribution, and more complex forms are not justified by this data set. Analysis of this data set shows no evidence of saturation of the polar cap potential for large |MF|. A simple unified description of the polar cap potential at all magnetic local times (MLT) and IMF, Φ(IMF, MLT) = −4.1 + 0.5 sin ((2π/24) MLT + 0.056 + 0.015 Byeff) (1.1 × 10–4 v2 + 11.1 B sin3 (θ/2)) kV, is generated, where Byeff is By (−BY) in the northern (southern) hemisphere. If IMF data is unavailable, the polar cap potential is well described by ΦA(Kp, MLT) = − 4.1 + 1/2 sin ((2π/24) MLT + ϕHM)(16.4 + 15.2 Kp) kV, where OϕHM is a small phase correction of (−0.054, −0.031, 0.040) for Heppner-Maynard convection patterns (BC, A, DE), respectively.

Electron-Scale Measurements of Magnetic Reconnection in Space

Probing magnetic reconnection in space Magnetic reconnection occurs when the magnetic field permeating a conductive plasma rapidly rearranges itself, releasing energy and accelerating particles. Reconnection is important in a wide variety of physical systems, but the details of how it occurs are poorly understood. Burch et al. used NASAs Magnetospheric Multiscale mission to probe the plasma properties within a reconnection event in Earths magnetosphere (see the Perspective by Coates). They find that the process is driven by the electron-scale dynamics. The results will aid our understanding of magnetized plasmas, including those in fusion reactors, the solar atmosphere, solar wind, and the magnetospheres of Earth and other planets. Science, this issue p. 10.1126/science.aaf2939; see also p. 1176 Magnetic reconnection is driven by the electron-scale dynamics occurring within magnetized plasmas. INTRODUCTION Magnetic reconnection is a physical process occurring in plasmas in which magnetic energy is explosively converted into heat and kinetic energy. The effects of reconnection—such as solar flares, coronal mass ejections, magnetospheric substorms and auroras, and astrophysical plasma jets—have been studied theoretically, modeled with computer simulations, and observed in space. However, the electron-scale kinetic physics, which controls how magnetic field lines break and reconnect, has up to now eluded observation. RATIONALE To advance understanding of magnetic reconnection with a definitive experiment in space, NASA developed and launched the Magnetospheric Multiscale (MMS) mission in March 2015. Flying in a tightly controlled tetrahedral formation, the MMS spacecraft can sample the magnetopause, where the interplanetary and geomagnetic fields reconnect, and make detailed measurements of the plasma environment and the electric and magnetic fields in the reconnection region. Because the reconnection dissipation region at the magnetopause is thin (a few kilometers) and moves rapidly back and forth across the spacecraft (10 to 100 km/s), high-resolution measurements are needed to capture the microphysics of reconnection. The most critical measurements are of the three-dimensional electron distributions, which must be made every 30 ms, or 100 times the fastest rate previously available. RESULTS On 16 October 2015, the MMS tetrahedron encountered a reconnection site on the dayside magnetopause and observed (i) the conversion of magnetic energy to particle kinetic energy; (ii) the intense current and electric field that causes the dissipation of magnetic energy; (iii) crescent-shaped electron velocity distributions that carry the current; and (iv) changes in magnetic topology. The crescent-shaped features in the velocity distributions (left side of the figure) are the result of demagnetization of solar wind electrons as they flow into the reconnection site, and their acceleration and deflection by an outward-pointing electric field that is set up at the magnetopause boundary by plasma density gradients. As they are deflected in these fields, the solar wind electrons mix in with magnetospheric electrons and are accelerated along a meandering path that straddles the boundary, picking up the energy released in annihilating the magnetic field. As evidence of the predicted interconnection of terrestrial and solar wind magnetic fields, the crescent-shaped velocity distributions are diverted along the newly connected magnetic field lines in a narrow layer just at the boundary. This diversion along the field is shown in the right side of the figure. CONCLUSION MMS has yielded insights into the microphysics underlying the reconnection between interplanetary and terrestrial magnetic fields. The persistence of the characteristic crescent shape in the electron distributions suggests that the kinetic processes causing magnetic field line reconnection are dominated by electron dynamics, which produces the electric fields and currents that dissipate magnetic energy. The primary evidence for this magnetic dissipation is the appearance of an electric field and a current that are parallel to one another and out of the plane of the figure. MMS has measured this electric field and current, and has identified the important role of electron dynamics in triggering magnetic reconnection. Electron dynamics controls the reconnection between the terrestrial and solar magnetic fields. The process of magnetic reconnection has been a long-standing mystery. With fast particle measurements, NASA’s Magnetospheric Multiscale (MMS) mission has measured how electron dynamics controls magnetic reconnection. The data in the circles show electrons with velocities from 0 to 104 km/s carrying current out of the page on the left side of the X-line and then flowing upward and downward along the reconnected magnetic field on the right side. The most intense fluxes are red and the least intense are blue. The plot in the center shows magnetic field lines and out-of-plane currents derived from a numerical plasma simulation using the parameters observed by MMS. Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA’s Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth’s magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.

Effects of the March 1989 solar activity

On Monday, March 6, 1989, a very large and complex sunspot group, Region 5395, rotated into view around the east limb of the Sun and quickly gained attention when it produced an X15/3B flare (N35, E69). The event began a period of high solar activity that lasted two weeks and had many important consequences at Earth and in near-Earth space. From March 6–19, Region 5395 produced 11 X-class and 48 M-class X ray flares. Prolonged proton events occurred that lasted several days and had an unusually high proportion of lower-energy particles. The solar activity produced an historically “great” magnetic storm, long-lasting Polar Cap Absorption events, and a major Forbush decrease, which is a decrease of the galactic cosmic ray flux observed at Earth. The ionosphere was greatly disturbed.

Identifying the plasmapause in IMAGE EUV data using IMAGE RPI in situ steep density gradients

plasmapause location observed by RPI is compared tothe location of the He + edge extracted from the closest-in-time EUVimage, a correlation coefficient of 0.83 is obtained. When the EUV He + edge location is taken as the average of two EUV measurements (one before and one after the RPI measurement), the correlation coefficient increases to 0.87. The high degree of correlation justifies the assumption that the He + edge coincides with the plasmapause. For eighteen cases inwhich the plasmasphere has no sharp outer boundary the intensity of the uncalibrated EUV images is compared with the electron number density extracted from the RPI data, and the lower sensitivity threshold of the EUV instrument is

Evidence for space weather at Mercury

Mercurys sodium atmosphere is known to be highly variable both temporally and spatially. During a week-long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind-magnetosphere interactions. We assume that photon-stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercurys magnetic moments are required to confirm the results.

Interhemispheric asymmetry of the high-latitude ionospheric convection pattern

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27-29, 1992. When the interplanetary magnetic field (IMF) Bz component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar-cap potential drops in the two hemispheres are similar. When Bz is positive and |By| > Bz, the convection configurations are mainly determined by By and they may appear as normal “two-cell” patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of |By|/Bz decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the “merging cell” potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The “lobe cell” provides a potential between 8.5 and 26 k V and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if |By| > |Bz|. To estimate the potential drop of the “viscous cells,” we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

IMF‐driven plasmasphere erosion of 10 July 2000

[1] On 10 July 2000, the IMAGE EUV imager observed erosion of the nightside plasmasphere that occurred in two bursts during 5-8 UT. The plasmapause radial velocity V PP at 2.4 MLT was extracted from the time sequence of EUV images. We show that intervals of V pp < 0 (i.e., erosion) are correlated with intervals of southward (S wd ) interplanetary magnetic field (IMF), if the solar wind and IMF data are time-delayed by 30 minutes (in addition to a 3.7-minute delay for propagation to the magnetopause). This suggests that coupling between the solar wind and the plasmapause, involving processes in the ionosphere and magnetotail, takes about 30 minutes. A 6:40 UT magnetosphere compression may have hurried the onset of the second erosion.

A Bx-interconnected magnetosphere model for Mercury

Abstract The access of solar wind plasma to the surface of Mercury depends on the magnetic fields in the vicinity of the planet. We present the structure of the Hermean magnetosphere obtained by the Toffoletto–Hill (J. Geophys. Res. 98 (1993)) model of a magnetically interconnected (“open”) magnetosphere modified for the size of Mercury and the strength of its magnetic field. We calculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and magnitude. These results are compared with existing sodium data obtained during a week-long period of observations in November 1997. Although quantitatively crude, the model gives a qualitative approach on how to interpret a good part of the sodium emissions. We conclude that increased ion-sputtering due to solar wind–magnetosphere interactions may explain the temporal and spatial variations of the sodium exosphere seen at Mercury. We predict that emissions should be stronger in the southern hemisphere for a positive Bx component, and in the northern hemisphere for a negative Bx. The Bz component regulates the size and position of the open field line region. More negative IMF Bz corresponds to more equatorial open flux regions.

First results from the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) is a 3 kHz to 3 MHz radio sounder, incorporating modern digital processing techniques and long electronically-tuned antennas, that is flown to large radial distances into the high-latitude magnetosphere on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Clear echoes, similar to those observed by ionospheric topside sounders, are routinely observed from the polar-cap ionosphere by RPI even when IMAGE is located at geocentric distances up to approximately 5 Earth radii. Using an inversion technique, these echoes have been used to determine electron-density distributions from the polar-cap ionosphere to the location of the IMAGE satellite. Typical echoes from the plasmapause boundary, observed from outside the plasmasphere, are of a diffuse nature indicating persistently irregular structure. Echoes attributed to the cusp and the magnetopause have also been identified, those from the cusp have been identified more often and with greater confidence.

Characteristics of ionospheric convection and field-aligned current in the dayside cusp region

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.