Stable Solar Wind Research Articles

Cusp structures: combining multi-spacecraft observations with ground-based observations

Abstract. Recent simultaneous observations of cusp structures with Polar, FAST and Interball revealed remarkably similar features at spacecraft crossing the cusp. Such stable cusp structures could be observed up to several hours only during stable solar wind conditions. Their similarities led to the conclusion that for such conditions large-scale cusp structures are spatial structures related to a global ionospheric convection pattern and not the result of temporal variations in reconnection parameters. With the launch of the Cluster fleet we are now able to observe precipitating ion structures in the cusp with three spacecraft and identical instrumentation. The orbit configuration of the Cluster spacecraft allows for delay times between spacecraft of about 45 min in crossing the cusp. The compact configuration of three spacecraft at about the same altitude allows for the analysis of cusp structures in great de-tail and during changing solar wind conditions. Cluster observations on 25 July 2001 are combined with SuperDARN radar observations that are used to derive a convection pattern in the ionosphere. We found that large-scale cusp structures for this Cluster cusp crossing are in agreement with structures in the convection pattern and conclude that major cusp structures can be consistent with a spatial phenomenon.Key words. Magnetospheric physics (energetic particles, precipitating, magnetopause, cusp arid and boundary layers; solar wind-magnetosphere interactions)

Transition to solar minimum at high solar latitudes: Energetic particles from corotating interaction regions

One of the key questions to be addressed during the solar maximum phase of the Ulysses mission has been the nature of the ubiquitous energetic particle populations observed at all helio‐latitudes from equator to poles. During the current, post‐maximum phase of its mission, Ulysses has encountered the return to more stable solar wind, high speed stream structure, leading to the formation of stream and/or corotating interaction regions, in addition to the coronal mass ejection associated transients. The analysis reported here presents the identification of the first CIR after the recent solar maximum recorded by Ulysses COSPIN/LET at high helio‐latitudes based on energetic particle composition data recorded in the ∼5–30 MeV/n range. The recurrent compression region appears during 3–6 solar rotations in 2002 and is observed from ≈52°N to ≈37°N heliographic latitude at heliocentric distances from ≈3.1 AU to ≈3.8 AU.

Spatial features observed in the cusp under steady solar wind conditions

Multispacecraft studies using Polar, Fast Auroral Snapshot (FAST), and Interball observations revealed that many cusp structures observed during stable solar wind conditions are spatial features. While individual cusp events appear to be very different from each other, multiple observations of the same cusp event during periods of stable solar wind conditions are remarkably similar. These spatial cusp structures have been interpreted as a result of the independent evolution of ion distributions from multiple reconnection sites (X‐lines) in both hemispheres. The location of reconnection sites depends critically on the direction of the interplanetary magnetic field. Thus individual cusp events should have similar cusp structures if they occur in the same magnetic local time sector during comparable solar wind conditions, especially during similar interplanetary magnetic field orientations. Data presented in this paper show just such an effect. For three individual cusp events observed by Polar and FAST, similar stepped ion structures are seen by both satellites passing though these large‐scale spatial structures.

Temporal versus spatial interpretation of cusp ion structures observed by two spacecraft

A series of nearly simultaneous cusp crossings by the Polar and Fast Auroral Snapshot (FAST) spacecraft are used to investigate the development of cusp structures such as sudden changes in the energy of cusp precipitating ions. While such changes are generally interpreted as temporal signatures, recent investigations show evidence that such features can also be interpreted as spatial structures. Our analysis of four events during stable solar wind conditions confirms that cusp structures observed by one satellite are remarkably similar to cusp features observed up to several hours later by a second satellite. Using the spatial separation of the Polar and FAST spacecraft, the cusp features could also be traced over several hours in magnetic local time. These similarities led to the conclusion that large‐scale cusp structures are spatial structures related to global ionospheric convection pattern set up by magnetic merging and not the result of temporal variations in reconnection parameters.

Corotating interaction regions

Corotating Interaction Regions are long lasting large-scale plasma structures generated in low and middle latitude regions of the heliosphere by the interaction of a stable fast solar wind stream with the surrounding slow solar wind. Associated with these plasma structures are recurrent MeV-ion events and recurrent decreases of the galactic and anomalous cosmic ray intensities. Prior to the Ulysses-mission the effect of corotating interaction regions could only be observed at low or moderate heliographic latitudes. The three-dimensional extent and their role in structuring the quiet heliosphere became clear after Ulysses high latitude observations had became available. Corotating interaction regions possibly play a role in determining solar modulation of galactic and anomalous cosmic rays during periods of low to moderate solar activity. In March 1996, April 1997, and June 1998 two workshops on Development and Effects of Corotating Interaction Regions were held at Elmau Castle, Germany, and at the International Space Science Institute in Bern, Switzerland, primarily based on new high latitude observations from Ulysses and considering as well in-ecliptic and outer-heliospheric spacecraft result. In this paper MeV-particle, galactic and anomalous cosmic ray as well as plasma observations from spacecraft in the inner heliosphere will be presented.

Observation of a correspondence between poleward moving auroral forms and stepped cusp ion precipitation

In a case study, we document for the first time a one‐to‐one association between three auroral forms which are separated in latitude and moving poleward and three intervals of almost constant, but different, low‐energy cutoffs in the ion precipitation in the cusp region (“stepped cusp”) precipitation. The ground observations are from Ny Alesund at 75° magnetic latitude (MLAT), and the precipitation pattern is observed by the DMSP F13 satellite as it crosses the noon meridian into the morning sector. The energy cutoffs increase with decreasing latitude. The auroral forms brighten up at different times but at practically the same MLAT (∼73°) and start fading 5 min later at ∼76° MLAT. Interplanetary conditions, monitored by WIND, show a stable, slow, and cold solar wind with an interplanetary magnetic field which points south and west (By > 0). Staircase ion precipitation signatures such as observed in this example have been successfully interpreted in terms of a sequence of reconnection bursts at the low‐latitude, dayside magnetopause. The time‐history of the auroral activity observed from the ground confirms this interpretation. We exclude the possibility that the auroral forms are triggered by changes in the interplanetary medium because the latter is very steady. In particular, the (low) dynamic pressure varies little about a mean value of 1.6 nPa.

Daily variation of geomagnetic field at low latitudes associated with stable solar wind flow

Mean diurnal variation ofH at low and equatorial latitudes is computed for days in the vicinity of passage of ‘quiet’ solar wind. It is shown that the prepassage magnitude of the diurnal variation is appreciably larger when compared to post-passage intervals at low latitudes but the difference vanishes in the electrojet region. It is suggested that the Sq current system moves towards dip equator immediately following quiet wind conditions relative to earlier periods. It is also shown that during conditions of stable solar wind, the solar wind proton density is inversely related to the electrojet strength, while at low latitudes outside the jet influence, there is no clear association.