[1] There has been considerable confusion in the literature about what mirror mode (MM), magnetic decrease (MD), and linear magnetic decrease (LMD) structures are and are not. We will reexamine past spacecraft observations to demonstrate the observational similarities and differences between these magnetic and plasma structures. MM structures in planetary magnetosheaths, cometary sheaths, and the heliosheath have the following characteristics: (1) the structures have little or no changes in the magnetic field direction across the magnetic dips; (2) the structures have quasiperiodic spacings, varying from ∼20 proton gyroradii (rp) in the Earth's magnetosheath to ∼57 rp in the heliosheath; and (3) the magnetic dips have smooth edges. Magnetosheath MM structures are generated by the mirror instability where β⊥/β∥ > 1 + 1/β⊥ (β is the plasma thermal pressure divided by the magnetic pressure). In general, the sources of free energy for the mirror instability are reasonably well understood: shock compression, field line draping, and, in the cases of comets and the heliosheath, also ion pickup. The observational properties of interplanetary MDs are as follows: (1) there is a broad range of magnetic field angular changes across them; (2) their thicknesses can range from as little as 2–3 rp to thousands of rp, with no “characteristic” size; and (3) they typically are bounded by discontinuities. The mechanism(s) for interplanetary MD generation is (are) currently unresolved, although at least five different mechanisms have been proposed in the literature. Tsurutani et al. (2009a) have argued against mirror instability for those MDs generated within interplanetary corotating interaction regions. Interplanetary LMDs are by definition a subset of MDs with small angular changes across them (θ < 10°). Are LMDs generated by the mirror instability or by another mechanism? Is it possible that there are several different types of LMDs involving different generation mechanisms? At the present time, no one knows the answers to these latter questions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA015913

Magnetosheath and heliosheath mirror mode structures, interplanetary magnetic decreases, and linear magnetic decreases: Differences and distinguishing features

An iterative method is developed by which one can 
calculate approximately the boundary of a magnetic field 
confined by a plasma. This method consists essentially of 
varying an assumed surface until the magnetic multipole 
moments of the currents, which would flow on that surface 
to balance the plasma pressure, cancel the corresponding 
moments of the magnetic sources within the surface. The 
method is applied to two problems. For a dipole source of moment M emu in a plasma of 
uniform pressure p dynes/cm^2 that does not penetrate the 
magnetic field, the approximate equation of the surface 
is r = 0.82615 M^(1/3) p^(-1/6)(1-0.120039α^2 - .004180α^4 - .001085α^6 + .000200α^8 - .000597α^(10) + .000326α^(12) - .000094α^(14)) cm, where α is the latitude in radians from the plane normal to M. The surface formed by a cold plasma of density N_0 and pair mass velocity M_t moving past a dipole of moment Me_y with a velocity –U_oe_z extends to infinity downwind. In a coordinate system (x, y, z) centered at the dipole, neutral points, where the surface is parallel to the wind direction, occur at the points (0, ±R_n, .27R_n), and other points on the surface are (0, 0, 1.02R_n), (0, ±2R_n, -∞) and (±1.97R_n, 0, -∞). R_n = 1.0035 (M/(M_tN_oU^2_o)^(-1/2)^(1/3) is about 9 earth radii for the solar wind case.

Calculation by a Moment Technique of the Perturbation of the Geomagnetic Field by the Solar Wind.

Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth's radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions. 

/pdf/dayside-transient-phenomena-and-their-impact-on-the-2g1krgha.pdf

Dayside Transient Phenomena and Their Impact on the Magnetosphere and Ionosphere

Magnetic holes (MHs), with a scale much greater than ρi (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), p ...

/pdf/observations-of-kinetic-size-magnetic-holes-in-the-51c75tf0j9.pdf

Observations of kinetic-size magnetic holes in the magnetosheath

Current density profiles in several types of interplanetary magnetic holes have been calculated using high-resolution Imp 6 magnetic field data (12.5 vector measurements/s), assuming that the currents flow in planar sheets and that the magnetic field varies only in the direction normal to the sheet. The planarity was verified in four holes which were observed by two suitably spaced spacecraft. The structure of the current sheets ranges from very simple in some holes to very complex in others. Four types of simple magnetic holes are discussed, in which B varies nearly monotonically on each side of the hole. In two of the holes, B varies in intensity but not in direction as a result of currents normal to B. In the other two holes, B changes in both magnitude and direction as a result of currents both normal and parallel to B. The observed structures are found to be qualitatively consistent with the models of Burlaga and Lemaire, which are based on self-consistent solutions of Vlasov's equation and Maxwell's equations. Examples of complex irregular magnetic holes are also presented, and they are shown to contain multiple current sheets in which currents flow parallel to one another at various angles with respect to B. There is no model of such magnetic holes at present.

Structure of current sheets in magnetic holes at 1 AU. [regions of low magnetic field intensity in the solar wind

. Interplanetary linear magnetic holes (LMHs) are structures in which the magnetic field magnitude decreases with little change in the field direction. They are a 10–30% subset of all interplanetary magnetic holes (MHs). Using magnetic field and plasma measurements obtained by Cluster-C1, we surveyed the LMHs in the solar wind at 1 AU. In total 567 interplanetary LMHs are identified from the magnetic field data when Cluster-C1 was in the solar wind from 2001 to 2004. We studied the relationship between the durations and the magnetic field orientations, as well as that of the scales and the field orientations of LMHs in the solar wind. It is found that the geometrical structure of the LMHs in the solar wind at 1 AU is consistent with rotational ellipsoid and the ratio of scales along and across the magnetic field is about 1.93:1. In other words, the structure is elongated along the magnetic field at 1 AU. The occurrence rate of LMHs in the solar wind at 1 AU is about 3.7 per day. It is shown that not only the occurrence rate but also the geometrical shape of interplanetary LMHs has no significant change from 0.72 AU to 1 AU in comparison with previous studies. It is thus inferred that most of interplanetary LMHs observed at 1 AU are formed and fully developed before 0.72 AU. The present results help us to study the formation mechanism of the LMHs in the solar wind.

/pdf/cluster-c1-observations-on-the-geometrical-structure-of-4bi9mrzjei.pdf

Cluster-C1 observations on the geometrical structure of linear magnetic holes in the solar wind at 1 AU

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Cluster-C1 observations on the geometrical structure of linear magnetic holes in the solar wind at 1 AU