High Time Resolution Magnetic Field Research Articles

EMIC waves observed at geosynchronous orbit under quiet geomagnetic conditions (Kp ≤ 1)

AbstractWe statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.

THEMIS observations of unusual bow shock motion attending a transient magnetospheric event

We present a multipoint case study of solar wind and magnetospheric observations during a transient magnetospheric compression at 23:19 UT on 15 October 2008. We use high time resolution magnetic field and plasma data from the THEMIS and GOES 11/12 spacecraft to show that this transient event corresponded to an abrupt rotation in the IMF orientation, a change in the location of the foreshock, and transient outward bow shock motion. We employ results from a global hybrid code model to reconcile the observations indicating transient inward magnetopause motion with the outward bow shock motion.

Are there current-sheet-like structures in the Earth's magnetotail as in the solar wind – results and implications from high time resolution magnetic field measurements by Cluster

Abstract. Recent studies of solar wind MHD turbulence show that current-sheet-like structures are common in the solar wind and they are a significant source of solar wind MHD turbulence intermittency. While numerical simulations have suggested that such structures can arise from non-linear interactions of MHD turbulence, a recent study by Borovsky (2006), upon analyzing one year worth of ACE data, suggests that these structures may represent the magnetic walls of flux tubes that separate solar wind plasma into distinct bundles and these flux tubes are relic structures originating from boundaries of supergranules on the surface of the Sun. In this work, we examine whether there are such structures in the Earth's magnetotail, an environment vastly different from the solar wind. We use high time resolution magnetic field data of the FGM instrument onboard Cluster C1 spacecraft. The orbits of Cluster traverse through both the solar wind and the Earth's magnetosheath and magnetotail. This makes its dataset ideal for studying differences between solar wind MHD turbulence and that inside the Earth's magnetosphere. For comparison, we also perform the same analysis when Cluster C1 is in the solar wind. Using a data analysis procedure first introduced in Li (2007, 2008), we find that current-sheet-like structures can be clearly identified in the solar wind. However, similar structures do not exist inside the Earth's magnetotail. This result can be naturally explained if these structures have a solar origin as proposed by Borovsky (2006). With such a scenario, current analysis of solar wind MHD turbulence needs to be improved to include the effects due to these curent-sheet-like structures.

Transients at the dusk side magnetospheric boundary: Surface waves or isolated plasma blobs?

We revisit Interball‐Tail and Magion‐4 observations of the dusk side magnetospheric boundary on February 15–16, 1996. The observed transient behavior of the boundary can be interpreted in terms of surface waves or as the manifestation of isolated magnetosheath plasma entities embedded in the magnetosphere. We examine the arguments for each of these interpretations with high time resolution magnetic field and plasma data and by exploiting the dual‐satellite nature of the observations. We find strong evidence for magnetic field and flow vortices near the magnetospheric boundary and hence for the existence of flux tubes with helicoidal field lines; such structures can be associated with both interpretations. The cross‐correlation between the dual satellite observations and the apparent periodicity strongly suggest a Kelvin‐Helmholtz surface wave, although other interpretations are not impossible. In any case, the observations for this particular event allow us to derive constraints on surface wave generation mechanisms and on scenarios that could account for the presence of isolated plasma elements in the magnetosphere.

Decay of Magnetic Field Irregularities Observed by Ulysses

We study the solar cycle, radial, and latitudinal dependence of the characteristics of magnetic field irregularities in the Heliosphere. The frequency of magnetic field discontinuities is determined, using high time resolution magnetic field observations by Ulysses, covering the time interval from 1992 to 2000. The quasi-linear scattering mean free path of particles is also calculated. These investigations aim at understanding/exploring transport properties of energetic charged particles in the Heliosphere. We find that the travel time of solar wind plasma from the Sun to the observer is the key parameter of the process, by controling the decay of the irregularities.

Fluctuations of the heliospheric magnetic field and the anisotropy of pickup ions

Pitch angle distribution of interstellar pickup ions at high heliographic latitude is studied. Based on high time resolution magnetic field observations by Ulysses during the south polar pass in 1994, we have determined the angular distribution of particles at the moment of ionization. Due to large deviations of the field from Parker's spiral, it was found that 13% of the freshly ionized particles stream away of the Sun in the solar wind frame. We have solved the transport equation of the pickup particles which included an anisotropic source term. Our model calculation shows that the effect of the source on the particle anisotropy is the dominant one, as opposed to the effect of the density gradient. Matching the results of the model with particle anisotropy observation by SWIGS aboard Ulysses, we have estimated a scattering mean free path of interstellar pickup protons λ = 0.25AU.

A multipoint determination of the propagation velocity of a sudden commencement across the polar ionosphere

We use magnetic field and riometer data from ground observatories in both the Arctic and Antarctic regions to characterize the high‐latitude propagation of a sudden storm commencement (SC) that occurred at 0901 UT February 21, 1994. (1) High time resolution magnetic field data from both hemispheres indicate extremely rapid propagation of the initial part of the SC signal at high latitudes. An initial inflection point was observed in the data from dawn sector stations in both polar caps nearly simultaneously (Δt <2 s) but ∼3 s earlier in the southern hemisphere. (2) Data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, with stations from 0130 to 0600 magnetic local time, indicate dispersive propagation of the preliminary impulse (PI) at speeds decreasing from ∼150 to ∼50 km/s, directly away from a source near or slightly poleward of the cusp, rather than along the auroral oval. (3) Plots of two‐dimensional, 20‐s resolution equivalent convection vectors in the Northern Hemisphere reveal the imposition and rapid propagation and decay of a short‐lived large‐scale flow pattern opposite to the normal dawn sector flow, followed by an intensification of the original pattern. However, the perturbed flows were not dominated by the localized, tailward propagating vortices predicted in some models of SC events. In both hemispheres, observations of the PI are consistent with the temporary imposition of a field‐aligned current pair antisymmetric about local noon in the polar ionosphere and with horizontal fast mode propagation of a pulse through the ionosphere. (4) Riometer signatures do not match the magnetic variations in either time or intensity, and they propagate with a much lower velocity (∼1 km/s). We infer that the initial magnetic signatures are generated by the arrival of Alfvén waves and associated Birkeland currents at the near‐cusp ionosphere, while the riometer signatures at these high latitudes are generated predominantly by interplanetary particles associated with the causative coronal mass ejection.

High resolution geomagnetic field observations at Terra Nova bay, Antarctica

he preliminary results obtained from the analysis in the micropulsation frequency range of high time resolution magnetic field data recorded at the Antarctic Italian geomagnetic observatory at Terra Nova Bay for 11 consecutive days in February 1994 are reported. The spectral index over the whole Pcl-Pc5 frequency range is of the order of 3.5 and its value significantly increases beyond about 50 mHz. Spectral peaks in the Pc3 frequency range are common, especially during the daytime hours, and are probably due to the direct penetration of upstream waves in the cusp region. From the local time distribution of the micro pulsation power, a signifi - cant activity enhancement around the local magnetic noon emerges, in agreement with previous observations. The analysis of the signal polarisation characteristics in the horizontal plane shows a predominant CW polarisation in the Pcl-Pc3 frequency ranges with the major axis of the polarisation ellipse in the first quadrant.

A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earth's magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP‐8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth‐Sun line. We note the presence of pulsations of ∼5‐hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self‐similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant‐α, force‐free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP‐8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5‐MeV ions while IMP‐8 was inside the cloud. These ions travel as a collimated, field‐aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well‐connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.

Observations of a new class of upstream waves with periods near 3 seconds

We have found a new class of ULF waves with periods near 3 s in the Earth's upstream region by examining the high time resolution magnetic field data from the ISEE spacecraft. These waves are observed in the part of the upstream region which is magnetically connected to the bow shock, but only when the solar wind plasma beta is high (greater than 1). The waves are always right‐handed, nearly circularly polarized in the spacecraft frame. The directions of the wave vectors are in the general direction of the average magnetic field, and the waves are convected downstream in the spacecraft frame. Our study of these waves has shown that they appear to be intrinsically left‐handed ion cyclotron waves in the plasma rest frame.

Multiple ground‐based and satellite observations of global Pi 2 magnetic pulsations

Four Pi 2 magnetic pulsations, observed on the ground at L = 1.2–6.9 in the interval from 2300 UT on May 22 to 0300 UT on May 23, 1985, provide new evidence of a global nature of Pi 2 pulsations in the inner (L ≲ 7) region of the magnetosphere bounded by the plasma sheet during quiet geomagnetic conditions. In the present study, magnetic data have been collected from stations distributed widely both in local time and in latitude, including conjugate stations, and from the AMPTE/CCE spacecraft located in the magnetotail. On the basis of high time resolution magnetic field data, the following characteristics of Pi 2 have been established: horizontal components, H and D, of the Pi 2 oscillate nearly antiphase and in‐phase, respectively, between the high‐ and low‐altitude stations in the midnight southern hemisphere. Both the H and D components of the Pi 2 have nearly in‐phase relationships between the nightside and the dayside stations at low latitude. The Pi 2 amplitude is larger at the high‐latitude station and decreases toward lower latitudes. The dominant periods of the Pi 2 are nearly identical at all stations. Although a direct coincidence between spacecraft‐observed and ground‐based global Pi 2 events does not exist for these events, the Pi 2 events are believed to be a forced field line oscillation of global scale, coupled with the magnetospheric cavity resonance wave in the inner magnetosphere during the substorm expansive phase.

A study of the coherence length of ULF waves in the Earth's foreshock

We have examined high time resolution magnetic field data for different separations of ISEE 1 and 2 in the Earth's ion foreshock region to study the coherence length of upstream ULF waves. By examining the correlation coefficients of the low frequency waves as a function of separation distance, we find that the correlation coefficient depends mainly on the separation distance of ISEE 1 and 2 transverse to the solar wind flow. It drops to about 0.5 when the transverse separation is about 1 RE, a distance much larger than the proton thermal gyroradius in the solar wind. Thus the coherence length of the low‐frequency waves is about one Earth radius, which is of the order of the wavelength, and is consistent with that estimated from the bandwidth of the waves.

Possible Kelvin‐Helmholtz waves driven by reconnection accelerated flows

We present new observations of highly steepened magnetic signals within the dayside magnetopause current layer, having a period of about 5 seconds. These signals are embedded within a reconnection region and we suggest they may be Kelvin‐Helmholtz waves driven unstable by the velocity shear associated with reconnection accelerated flows. It is due to the low magnetic field strength and consequent high Alfvén Mach number that the signals are confined to the magnetopause. Our data consist of high time resolution magnetic field and plasma measurements from the AMPTE‐UKS satellite.

Slow mode shocks in the Earth' magnetotail: ISEE‐3

High time resolution magnetic field measurements adjacent to the boundary of the distant magnetotail plasma sheet have been analyzed. The nature of the changes in the field between the lobe and the plasma sheet is consistent with the boundary being a slow mode shock. The crossing times are relatively long (∼30 sec) implying a shock thickness estimated to be approximately 10³ km. An increase in the entropy of the electrons is consistent with dissipation at the shock. Reasonable shock normal directions are derived and may be used to determine the location of the spacecraft relative to the reconnection or merging region. It is suggested that a foreshock exists in which upstream wave and particle phenomena are occurring.