Goran Marklund

Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals

The characteristics of the fine scale electric field associated with the westward traveling surge and large-scale auroral spirals and surges are investigated using high-resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field-aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high-energy electron precipitation but very weak electric fields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in-between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.

Progress on relating optical auroral forms and electric field patterns

Ultraviolet auroral imagery and in situ electric field measurements from Freja are presented and used to identify signatures of the perpendicular electric field associated with three basic arc configurations. The auroral imagery is essential in identifying the two- dimensional characteristics of the discrete auroral forms for which electric field signatures are examined. The high-resolution data show the differences between the electric field signatures of the quiet and disturbed arcs (spirals) occurring as a single arc or as a part of a series of parallel arcs within the dusk and premidnight auroral distribution. The scale size of the arcs is given by their thickness, typically x7f 100 km. Common electric field signatures, such as a reduction in the convection electric field in the region of the arc and enhancements of the convection electric field at the boundaries of the arc, are observed. The electric field reductions dominating the signatures of quiet arcs occurring as a single arc indicate that these arcs are characterized with a weak field-aligned current. As inferred from the electric field enhancements dominating the signature of quiet arcs which are a part of a series of arcs, these arcs are characterized with a strong field-aligned current. Features which distinguish the quiet arcs from the disturbed arcs are the quasi-periodic fluctuations in thedisturbed arcs electric field signature. The turbulence in the electric field indicates that shear structures and shear-driven plasma instabilities are involved in the formation of auroral spirals.

Boundary layer polarization and voltage in the 14 MLT region

Viking midaltitude observations of ions and electrons in the postnoon auroral region show that field-aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi-electrostatic field-aligned acceleration process below Viking altitudes, i.e., below 10,000–13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside “ion inverted Vs.” The total transverse potential throughout the entire region near the ion inverted Vs is generally much higher than the field-aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum upgoing ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field-aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field-aligned potential structures connect to the inner part of an MHD dynamo in the low-latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in the 0.5–10 kV range.

A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases

One of the (DAW 9( substornts is investigated in this pa. per using the database reported by Hones et al. and supplelnented with magnetogram inversion technique (MIT) 2 data.. These latter have provided information about the dynamics of the open ta. il magnetic flux, current syst. ems in the ionosphere, and the size a. nd dyna, lnics of the current wedge. We have identified the growth, expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919- 0935) (?T, i.e., between the growth and expansion phases, indicating the concurrent development of the substorln onset and corresponding instabilities in the innermost current sheet,, and slna,ll-scale cross-tail current disruptions without the open tail reconnection. In addition to signatures of small-scale dipolariza, tion, an increase of the open tail magnetic flux, and a current system of the type close to D P 2 were observed a,t. (0919-0935) tIT, which is more likely to suggest predominance of the tail-stretching process than magnetic colla.pse. This fact was interpreted in letres of a releva, nt simple model a.s a signa.ture of the growth of the energy input hom the solar wind which ensures the observable disturbance power. Hence the disturba,nce a,t (0919-0935) UT was more likely a driven one than a,n mfioading one. The aforementioned signa,tures make it possible t.o identify the interva.l (0919-0935) UT a,s the pha.x7f_x7fe of multiple onsets or (equivalently) the first a, ctive phase, which was previously defined by Mishin (1991, and references therein) a,s one of the four standard phases of a typical substorln (in addition to the expansion phase). Thus the ca. se st.x7ftdy supports the substorm scenario with two active phases and, a. ccordingl.x.., with two different kinds of physics. This case study illustrates also the inforlnativity of MIT 2 data and their ability to effectively complement the database tra, ditionally used in substorm studies.

Magnetospheric Multiscale Mission observations and non-force free modeling of a flux transfer event immersed in a super-Alfvenic flow

We analyze plasma, magnetic field, and electric field data for a flux transfer event (FTE) to highlight improvements in our understanding of these transient reconnection signatures resulting from h ...

MMS Observations of Reconnection at Dayside Magnetopause Crossings During Transitions of the Solar Wind to Sub‐Alfvénic Flow

We present MMS observations during two dayside magnetopause crossings under hitherto unexamined conditions: (i) when the bow shock is weakening and the solar wind transitioning to sub-Alfvenic flow ...