L. J. Lanzerotti

Cusp latitude magnetic impulse events: 1. Occurrence statistics

Magnetic impulse events were selected by a computer algorithm procedure from magnetic records obtained at the near cusp latitude conjugate stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica. The algorithm was constructed to select large (≳ 50 nT in the vertical component of the magnetic field), short lived (6 to 12 min) events. These events were found to be highly localized in the 06 to 18 LT sector at the two stations. A strong minimum in occurrence was found during hour 11 LT. The field changes associated with these events can be interpreted as due to an approximately half-cycle, odd-mode, Alfven wave along a near-magnetopause flux tube. From the vertical magnetic deflections of the impulse events the directions of field-aligned currents into the conjugate ionospheres were inferred. In the morning LT sector, field-aligned currents were directed into the ionospheres, while in the afternoon LT sector, field-aligned currents were directed out of the ionospheres. These findings are comparable with the statistical results for quasi-stationary field-aligned currents and suggest that at the times of these events, Iqaluit and South Pole are at a higher effective magnetic latitude. The average deflection in the vertical component for the events was measured to be ∼ 95 nT. From this the magnitude of the average field-aligned currents was calculated to be J∥ ∼ 2 × 10−7 A/m².

The Peak Flux Distribution of Solar Radio Bursts

We have investigated the peak flux distribution of 40 years of solar radio burst data as a function of frequency and time over a wide range of frequencies. The bursts were reported by observing stations around the world during 1960-1999, as compiled by the National Geophysical Data Center (NGDC) of the National Oceanic and Atmospheric Administration (NOAA). This period covers three full and two partial solar cycles. We have analyzed the data set to find correction factors for missed events, and find evidence that nearly half of the events were missed by the worldwide network. We obtain power-law fits to the differential (density) (dN/dS in events sfu-1) and cumulative [N(> S) in events] distributions as a function of frequency, time, and phase of the solar cycle. The typical power-law index, ~-1.8, is similar to that found in many hard X-ray studies. The average waiting time between bursts with flux density exceeding 1000 sfu was found to be 6 days at solar maximum, and 33 days at solar minimum. Taking account of missed events, the expected waiting time decreases to 3.5 and 18.5 days, respectively. Bursts of this flux level can cause problems with wireless communication systems. We present tables of fit parameters that can be used to find burst occurrence rates in a number of frequency ranges. We find no significant variation of power-law index from one solar cycle to the next, or with phase of the solar cycle, but we do find significant changes of power-law index with frequency.

Latitude and longitude dependence of storm time Pc 5 type plasma wave

The recent study of a storm time Pc 5 plasma wave event by Lanzerotti, Fukunishi, Lin, and Cahill is extended by the analysis of recently available high-latitude ground-based magnetometer data and hot plasma particle (∼1–40 keV) distribution data from a satellite in the magnetosphere. These additional data suggest that an odd mode shear Alfven wave (period ∼7 min) was excited by a high β drift instability at a high-altitude gradient region of the hot plasma distribution. The region of occurrence of the instability was characterized by a cold plasma density of ∼100 cm−3.

Galileo‐measured depletion of near‐Io hot ring current plasmas since the Voyager epoch

The first mass-discriminated, hot ion distribution moments (pressure, energy intensity) are determined for hot >50-keV ions in Jupiters inner magnetosphere at the outer edge of Ios plasma torus by using the Galileo energetic particle detector (EPD) data. These hot plasmas were significantly depleted during the Galileo encounter in 1995 as compared with the Voyager epoch of 1979. The depletion of the hot ions is apparently caused by enhanced charge exchange losses of hot ions, perhaps associated with enhanced emissions of neutral gases from the volcanoes of Io. Such neutral gas enhancements could simultaneously explain increases, reported elsewhere, in the densities of the cooler Io torus plasmas. The hot plasma changes may explain why radial transport interchange turbulence has been observed by Galileo in the Io torus regions, whereas such turbulence was not apparent during the Voyager encounters in 1979. The hot ion depletion could also play a role in explaining the apparent differences between the Jovian auroral configuration observed in recent years by the Hubble space telescope and ground observers and the configuration observed by Voyager. This possibility is much less certain, however.

Studies of spacecraft charging on a geosynchronous telecommunications satellite

Abstract Reported here are the results of some initial analysis of space plasma charging of two charge plate sensors that are flying on the AT&T Telstar 402R geosynchronous communications satellite (89° W longitude). Charging occurs on the nightside of Earth during conditions when the plasma sheet in the magnetotail moves earthward under an enhanced cross-tail electric field. The March–June 1996 interval examined occurs during solar minimum conditions and is geomagnetically quiet. Some statistical distributions of the magnitudes of the observed charging show that the normal potential of the charge plate on the anti-Earth panel of the satellite is about −150 V. The charge plate on the north transponder panel is found to have a normal potential of about −150 V during the March equinox period and about −80 V during the June solstice when it is under continual solar illumination. The largest natural charging events measured during this geomagnetically quiet interval should not be of serious design concern.

Reappearance of recurrent low‐energy particle events at Ulysses/HI‐SCALE in the northern heliosphere

In the high-latitude northern hemisphere of the heliosphere, the Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energies (HI-SCALE) energetic particle detectors on Ulysses have measured quasi-recurrent ∼26-day increases of 40-65 keV electrons during 11 solar rotations from October 1995 through July 1996. They do not appear on all rotations, but when they do, they are associated with increases in 0.5-1.0 MeV protons (preceding the electron increases by several days) and sometimes with decreases in galactic cosmic rays. The northern recurrences form two series shifted half a solar rotation with respect to each other, unlike the very regular and more intense series of 21 recurrences observed by the same instrument throughout the middle-to-high-latitude southern hemisphere from mid-1993 to the beginning of 1995. Correlated energetic particle measurements from IMP 8 at Earth and Voyagers 1 and 2 at 42-62 AU establish that recurrent events during this period were intrinsically stronger in the southern heliosphere than in the north. The variability of the northern recurrences is attributed, using a generalization of the model of Fisk [1996], to temporal changes during 1966 in the near-Sum polar magnetic field configuration. These changes would affect the connection of Ulysses via magnetic field lines to the corotating interaction regions at lower latitudes >10 AU beyond the spacecraft, where the low-energy particles are accelerated and the galactic cosmic rays are modulated. The observed evolution of the northern polar coronal structure, as revealed in FeXIV (5303A) synoptic maps and confirmed by FeXII (195h) images from the Extreme Ultra-Violet Imaging Telescope on the SOHO spacecraft, is just that which is required, according to the model, to explain the evolution of the low-energy particle recurrences as observed by Ulysses/HI-SCALE in the northern heliosphere.

Rotationally driven 'zebra stripes' in Earth's inner radiation belt.

Structured features on top of nominally smooth distributions of radiation-belt particles at Earth have been previously associated with particle acceleration and transport mechanisms powered exclusively by enhanced solar-wind activity. Although planetary rotation is considered to be important for particle acceleration at Jupiter and Saturn, the electric field produced in the inner magnetosphere by Earth’s rotation can change the velocity of trapped particles by only about 1–2 kilometres per second, so rotation has been thought inconsequential for radiation-belt electrons with velocities of about 100,000 kilometres per second. Here we report that the distributions of energetic electrons across the entire spatial extent of Earth’s inner radiation belt are organized in regular, highly structured and unexpected ‘zebra stripes’, even when the solar-wind activity is low. Modelling reveals that the patterns are produced by Earth’s rotation. Radiation-belt electrons are trapped in Earth’s dipole-like magnetic field, where they undergo slow longitudinal drift motion around the planet because of the gradient and curvature of the magnetic field. Earth’s rotation induces global diurnal variations of magnetic and electric fields that resonantly interact with electrons whose drift period is close to 24 hours, modifying electron fluxes over a broad energy range into regular patterns composed of multiple stripes extending over the entire span of the inner radiation belt.

Magnetic impulse event: A detailed case study of extended ground and space observations

Analysis of conjugate data from extended magnetometer networks in northern and southern high latitudes is used to elucidate the initiation and the evolution of a magnetic impulse event (MIE) on June 6, 1997. In addition, data from all-sky imagers, imaging riometers, and Super Dual Auroral Radar Network radars in Antarctica are investigated to confirm the energy content, motion, and electrical current structure of the MIE. The MIE was accompanied by traveling convection vortices (TCVs) that began at ∼10 MLT and moved eastward (toward dusk) and slightly equatorward at 1-3 km/s across the noon meridian with north-south conjugacy. The MIE had upward field-aligned currents with soft electron precipitation that was located near the trailing edge of the Hall current loop. During the MIE interval the interplanetary magnetic field (IMF) was directed strongly outward from the Sun (B x = -5 nT), with a slightly positive (1-2 nT) B z , and a nearly zero By. Since abrupt solar wind pressure changes are unlikely under this IMF orientation (and none was, in fact, observed), classical mechanisms for MIE generation, such as a pressure pulse or dayside reconnection, are excluded. It is speculated that an abrupt IMF cone angle change from 60° to 20°, ∼30 min prior to the MIE onset, may have been an indirect trigger of this event via the interaction between the solar wind and the bow shock.

Large-scale electric field measurements on the Earth's surface : a review

There exist only a few reported measurements of quasi-stationary (near dc) electric potentials over very large spatial scales (hundreds of kilometers or more) on the Earths surface. Such measurements have typically been made using unpowered submarine telecommunications cables. The measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents. In addition, there are possible interpretational problems from pervasive, poorly understood, low-frequency electric fields induced by ocean water motion through the Earths stationary magnetic field. Nevertheless, estimates of the magnitude of the electric field computed from large-scale potential difference measurements, made principally to date in the Pacific Ocean, can be used to place a limit on the size of the toroidal magnetic field at the core-mantle boundary under certain conditions on the Earths electrical conductivity profile. Thus, large-scale electric potential measurements can serve as an adjunct probe of the Earths dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude.

Solar energetic particles inside a coronal mass ejection event observed with the ACE spacecraft

Abstract In this work, solar flare energetic particle fluxes (E e ⩾38 keV ) observed by the EPAM experiment aboard ACE are utilized as diagnostics of the large-scale structure and topology of the interplanetary magnetic field (IMF) embedded within a well-identified interplanetary coronal mass ejection (ICME). The still controversial question of whether the detected ICME structure has been detached from the solar corona or is still magnetically anchored to it is addressed. The observation of two impulsive solar flare electron events inside the ICME suggests that field lines in this ICME are rooted at the Sun. From the time evolution of the angular distributions of the particle intensities we infer that the observations are consistent with the magnetic topology of a magnetic bottle between a magnetic mirror located at the Sun and a magnetic constriction upstream from ACE formed by the convergence of open field lines that reflects the outgoing electrons. The magnetic mirror strength is calculated in one case based upon the local IMF observations and the electron event onset characteristics. A magnetic field enhancement observed by ACE in the downstream region of the CME-driven shock is identified as the agent responsible for the mirroring of the energetic electrons.