Small-scale Magnetic Flux Ropes Research Articles

Local and nonlocal geometry of interplanetary coronal mass ejections: Galactic cosmic ray (GCR) short‐period variations and magnetic field modeling

Energetic galactic cosmic ray (GCR) particles, arriving within the solar system, are modulated by the overall interplanetary field carried in the solar wind. Localized disturbances related to solar activity cause further reduction in intensity, the largest being Forbush decreases in which fluxes can fall ∼20% over a few days. Understanding Forbush decreases leads to a better understanding of the magnetic field structure related to shock waves and fast streams originating at the Sun since the propagation characteristics of the GCR probe much larger regions of space than do individual spacecraft instruments. We examined the temporal history of the integral GCR fluence (≥100 MeV) measured by the high‐sensitivity telescope (HIST) aboard the Polar spacecraft, along with the solar wind magnetic field and plasma data from the ACE spacecraft during a 40‐day period encompassing the 25 September 1998 Forbush decrease. We also examined the Forbush and (energetic storm particles) ESP event on 28 October 2003. It is the use of HIST in a high‐counting‐rate integral mode that allows previously poorly seen, short‐scale depressions in the GCR fluxes to be observed, adding crucial information on the origin of GCR modulation. Variability on time scales within the frequency range 0.001–1.0 mHz is detected. This paper concentrates on investigating four simple models for explaining short‐term reductions in the GCR intensity of both small and large amplitude. Specifically, these models are a local increase in magnetic scattering power, the passage of a shock discontinuity, and the passage of a tangential discontinuity or magnetic rope in the solar wind plasma. Analysis of the short‐scale GCR depressions during a test period in September through October 1998 shows that they are not correlated with changes in magnetic scattering power or fluctuations in solar wind speed or plasma density. However, magnetic field and plasma data during the test period of Forbush decrease strongly suggest the presence of an interplanetary coronal mass ejection (ICME). Use of a non‐force‐free magnetic rope model in conjunction with the energetic particle data allows modeling of the geometry of the ICME in terms of a magnetic cloud topology. It is only this cloud configuration that allows a satisfactory explanation of the magnitude of the Forbush event of 25 September 1998. Calculations made during the test period point to short‐scale GCR depressions being caused by either small‐scale magnetic flux rope structures or possibly tangential discontinuities in the solar wind.

Small‐scale magnetic flux ropes in the solar wind

Small‐scale magnetic flux ropes have been discovered in the solar wind at 1 AU in observations from the IMP 8 and WIND spacecraft. These small magnetic structures (diameter of 270 RE, on average) have some similar properties to magnetic clouds (diameters of 0.2–0.3 AU or about 6000–8000 RE), which are well known large‐scale magnetic flux ropes, but have durations of 10s of minutes as opposed to many hours or days for most magnetic clouds. The presence of these small helical field structures suggests that solar wind flux ropes may have a wide‐range of scale sizes, or possibly have a bimodal size distribution, and are perhaps more common than previously estimated. Similarities and differences with magnetic clouds will be discussed. We suggest that these small scale magnetic flux ropes are signatures of magnetic reconnection in the solar wind as opposed to in the solar corona.

Ulysses observation of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection

A well‐defined, small‐scale (≈0.05 AU) magnetic flux rope was observed by Ulysses at about 5 AU in close proximity to a heat flux dropout (HFD) at the heliospheric current sheet (HCS). This magnetic flux rope is characterized by a rotation of the field in the plane approximately perpendicular to the ecliptic (and containing the Sun and the spacecraft) and with a magnetic field maximum centered near the inflection point of the bipolar signature. The edges of the flux rope are well defined by diamagnetic field minima. A bidirectional electron heat flux signature is coincident with the magnetic flux rope structure. The event occurred during a time of slightly increasing solar wind speed, suggesting that the field and plasma were locally compressed. Unlike most coronal mass ejections/magnetic clouds, this event is characterized by high proton temperatures and densities, high plasma beta, no significant alpha particle abundance increase, and a small radial size. We interpret these observations in terms of multiple magnetic reconnection of previously open field lines in interplanetary space at the HCS. Such reconnection produces a U‐shaped structure entirely disconnected from the Sun (which we associate with the HFD), a closed magnetic flux rope (which we associate with the counterstreaming electron event), and a closed magnetic loop or tongue connected back to the Sun at both ends. These observations suggest that magnetic reconnection, and its changes to magnetic field topology, can occur well beyond the solar corona in interplanetary space.