ALBERT

Description: [1]  Neutron monitors have recorded the flux of high energy Galactic cosmic rays for more than half a century. During the recent prolonged, deep minimum in solar activity many sources indicate that modulated Galactic cosmic rays have attained new space-age highs. However reported neutron monitor rates are ambiguous; some record new highs while others do not. This work examines the record of 15 long-running neutron monitors to evaluate cosmic ray fluxes during the recent extraordinary solar minimum in a long-term context. We show that ground-level neutron rates did reach a historic high during the recent solar minimum, and we present a new analysis of the cosmic ray energy spectrum in the year 2009 versus year 1987. To do this we define a reference as the average of eight high-latitude neutron monitors, four in the northern hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the southern hemisphere (Kerguelen, McMurdo, Sanae, Terre Adelie). Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges. With our correction all stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. These increases are shown to be consistent with spacecraft observations.

Description: [1]  We present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric footpoint of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the pre-midnight local time sector and that solar illumination at the footpoint (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is ~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated footpoints. The beam energy during dark solar minimum conditions is a factor of ~3 more than during sunlit minimum conditions. The ‘area’ covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

Description: [1]  In this paper we describe and quantify the energy transfer, flow and distribution. Our high-resolution data-set covers 13 years of OMNI, SuperMAG and Kyoto data. We employ what we consider to be the best estimates for energy sinks, and relate these to SuperMAG indices for better coverage and spatial resolution. For the energy input we have used the method of dimensional analysis [ Vasyliunas et al ., 1982] that is presented in unit power and makes it appropriate for energy analysis. A cross-correlation analysis parameterizes the magnetospheric response on the solar wind parameters during a wide range of conditions, ranging from substorms and storms up to a decade. The determined functional form is then evaluated and scaled using superposed epoch analysis of geomagnetic storms, revealing that the effective area of interaction can not be considered static. Instead we present a dynamic area which depends to the first order on the cube of the IMF B z component. Also, we find that for longer time periods this area must be increased compared to the area used for geomagnetic storms. We argue that some of the terms in the energy coupling function are contributory to describing magnetosheath conditions, and discuss how our coupling function can be related to Maxwell stress components. Also, we quantify the relative importance of the different energy sinks during substorms, geomagnetic storms and long time series, and present the coupling efficiency of the solar wind. Our energy coupling functions is compared with the ɛ parameter [ Akasofu and Perreault , 1978] and performs better for almost any event.

Description: [1]  Plasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow-aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground-based auroral imager observations to identify flow bursts beyond 45 R E downtail and corresponding auroral forms. We find that quiet-time flows are directed dominantly earthward with a one-to-one correspondence with PBIs. Flow bursts during the substorm recovery phase, and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Pre-substorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant tail neutral line (DNL) or tailward-retreated near-Earth neutral line (NENL) rather than from the nominal NENL location in the mid-tail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion-phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

Description: [1]  The solar eclipse on 15 January 2010 traversed Asia and completed its travel on the Shandong Peninsula in China at sunset. Two vertical-incidence ionosondes at Wuhan and Beijing and the oblique-incidence ionosonde network in North China were implemented to record the ionospheric response to the solar eclipse. Following the initial electron density decrease caused by the eclipse, the ionosphere was characterized by a strong pre-midnight enhancement, and a subsequent ionospheric decay, and a ~10 hour later post-midnight enhancement. Neither geomagnetic disturbance occurred during the eclipse day, nor did obvious nighttime peak appear for the ten-day mean of the F2-layer critical frequency ( fo F2). The electron density profilogram of the Beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse. The two-dimensional differential fo F2 maps present the regional variations of the nighttime electron density peaks and decay. Both the pre- and post-midnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward. The different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux. The ionospheric decay following the pre-midnight enhancement is also considered as a consequence of the eclipse shade.

Description: [1]  We study a statistics of ∂  B z /∂  x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 R E downtail. After 2005 spacecraft separation allowed to measure directly this derivative of B z along the tail axis. The near-tail events (within 14 R E ) exhibited a straight decrease of an initially large positive ∂  B z /∂  x to ∼ 1–2 nT/ R E . In the more stretched middle tail, usually the small | ∂  B z /∂  x | 〈 0.5 − 1 nT/ R E had no clear trend and fluctuated around zero with time scales 5–15 min. In general, negative ∂  B z /∂  x were ubiquitous. At some onsets larger negative ∂  B z /∂  x  〈 − 1 nT/ R E were associated with transient dipolarizations, propagating Earthward. There was no clear association of local plasma sheet activity onset with any value of ∂  B z /∂  x . We discuss relation of observations and recent modeling results.

Description: [1]  ELF/VLF radio waves are difficult to generate with conventional antennas. Ionospheric HF heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet, creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW HAARP facility near Gakona, Alaska, and provide a baseline reference of ELF/VLF generation capabilities with HF heating. Between February 2007 and August 2008, HAARP was operated on close to 100 days for ELF/VLF wave generation experiments, at a variety of ELF/VLF frequencies, seasons and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500-3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self-consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency-time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker, but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20-30 dB. Generation strengthens by ~5 dB during the first ~30 minutes of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching250 km. The median power generated by HAARP and injected into the waveguide is ~0.05-0.1 W in this base-line configuration (vertical beam, 3.25 MHz, amplitude modulation), but may have generated hundreds of Watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

Description: [1]  An inversion technique for estimating the properties of the magnetospheric plasma from the harmonic frequencies of the toroidal standing Alfvén waves has been used to derive the global equatorial mass density covering radial distances from 4 to 9 Earth radii ( R E ), within the local time sector spanning from 0300 to 1900 hours. This broad range of L shell extending to the outer magnetosphere allows us to examine the local time and radial dependence of the quiet-time equatorial mass density during solar minimum and thereby construct a global distribution of the equatorial mass density. The toroidal Alfvén waves were detected with magnetometers on the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) during the nearly 5 year interval from August 1984 to January 1989 and on the Geostationary Operational Environmental Satellites (GOES) (10, 11 and 12) for 2 years from 2007 to 2008, both of which were operating during solar minimum years. The derived equatorial mass density, ρ eq , at geosynchronous orbit (GEO) monotonically increases with increasing magnetic local time (MLT) from the nightside towards the dusk sector. At other radial distances, ρ eq has the same MLT variation as that of GEO, while the magnitude logarithmically decreases with increasing L value. An investigation of the Dst and Kp dependence shows that the median value of ρ eq varies little in the daytime sector during moderately disturbed times, which agrees with previous studies. ρ eq calculated from the F 10.7 dependent empirical model shows good agreement with that of CCE but overestimates that of GOES probably due to the extreme solar cycle minimum in years 2007–2008.

Description: [1]  In this paper we compare observations of the high latitude cusp from DMSP data to simulations conducted using the Lyon-Fedder-Mobarry (LFM) global magnetosphere simulation. The LFM simulation is run for the 31 Aug 2005 to 02 Sep 2005 moderate storm, from which the solar wind data exhibits a wide range of conditions that enable a statistical representation of the cusp to be obtained. The location of the cusp is identified using traditional magnetic depression and plasma density enhancement at high altitude. A new diagnostic using the parallel ion number flux is also tested for cusp identification. The correlation of the cusp latitude and various solar wind IMF coupling functions is explored using the three different cusp identification methods. The analysis shows 1) the three methods give approximately the same location and size of the simulated cusp at high altitude; 2) the variations of the simulated cusp are remarkably consistent with the observed statistical variations of the low-altitude cusp. In agreement with observations a higher correlation is obtained using other solar wind coupling functions such as the Kan-Lee electric field. The MLT position of the simulated cusp is found to depend upon the IMF By component, with a lower linear correlation. The width of the simulated cusp in both latitude and MLT is also examined. The size of the cusp is found to increase with the solar wind dynamic pressure with saturation seen when the dynamic pressure is greater than 3 nPa.

Description: [1]  Daily profiles of phase measurements as observed on fixed VLF-paths generally show a transient phase advance, followed by a phase delay, for about 90 minutes after sunrise hours. This is indicative of a reflecting ionospheric C-region developing along the terminator line at an altitude below the normal D-region. The suggested occurrence of a C-region is consistent with rocket measurements made in the 1960's, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980's. In order to correctly describe the properties of the phase effect associated with the presence of a C-region, it is important to understand the subionospheric propagation characteristics of the VLF-paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrow-band phase effect, and determined a parameter associated with the appearance of the C-region at sunrise hours observed by receivers from the South America VLF Network (SAVNET). Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the TIMED-SABER satellite instrument, which we assume to be related to the Winter Anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one-to-one correspondence was found for transient events.