Van Allen Zone Research Articles

Drift echoes and flux oscillations: A signature of prompt and diffusive changes in the radiation belts

Prompt creation of new multi-MeV electron and proton radiation belts in the normally benign slot region between inner and outer Van Allen zones was observed by the short-lived Combined Release and Radiation Effects Satellite (CRRES) mission almost 30 years ago. Flux oscillations at lower energies coincident with ULF wave measurements and consistent with drift resonance were also a feature of the CRRES observations. The next mission designed to study the radiation belts in similar detail from a near equatorial plane orbit was the Van Allen Probes launched in August 2012. These twin spacecraft in similar orbits observed many milder events which produce drift echoes indicative of injection over a limited range of longitude on less than an electron drift time. Related substorm injection drift echoes were also observed by Van Allen Probes with much higher energy resolution than previously. Along with other flux oscillations associated with Ultra Low Frequency (ULF) waves, these events form a class which contributes to the overall radial transport and energization of radiation belt electrons as well as protons. This paper focuses on the greatly improved measurements from the Van Allen Probes relative to the CRRES era. These measurements illuminate our understanding of the importance of flux oscillations as a signature of radial transport relative to other energization and loss processes affecting the radiation belts.

Multiyear Measurements of Radiation Belt Electrons: Acceleration, Transport, and Loss.

In addition to clarifying morphological structures of the Earth's radiation belts, it has also been a major achievement of the Van Allen Probes mission to understand more thoroughly how highly relativistic and ultrarelativistic electrons are accelerated deep inside the radiation belts. Prior studies have demonstrated that electrons up to energies of 10 megaelectron volts (MeV) can be produced over broad regions of the outer Van Allen zone on timescales of minutes to a few hours. It often is seen that geomagnetic activity driven by strong solar storms (i.e., coronal mass ejections, or CMEs) almost inexorably leads to relativistic electron production through the intermediary step of intense magnetospheric substorms. In this study, we report observations over the 6‐year period 1 September 2012 to 1 September 2018. We focus on data about the relativistic and ultrarelativistic electrons (5 MeV) measured by the Relativistic Electron‐Proton Telescope sensors on board the Van Allen Probes spacecraft. This work portrays the radiation belt acceleration, transport, and loss characteristics over a wide range of geomagnetic events. We emphasize features seen repeatedly in the data (three‐belt structures, “impenetrable” barrier properties, and radial diffusion signatures) in the context of acceleration and loss mechanisms. We especially highlight solar wind forcing of the ultrarelativistic electron populations and extended periods when such electrons were absent. The analysis includes new display tools showing spatial features of the mission‐long time variability of the outer Van Allen belt emphasizing the remarkable dynamics of the system.

Highly relativistic radiation belt electron acceleration, transport, and loss: Large solar storm events of March and June 2015.

Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection‐driven event occurred with a Dst (storm time ring current index) value reaching −223 nT. On 22 June 2015 another strong storm (Dst reaching −204 nT) was recorded. These two storms each produced almost total loss of radiation belt high‐energy (E ≳ 1 MeV) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10 MeV in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong “butterfly” distributions with deep minima in flux at α = 90°. However, despite strong driving of outer zone earthward radial diffusion in these storms, the previously reported “impenetrable barrier” at L ≈ 2.8 was pushed inward, but not significantly breached, and no E ≳ 2.0 MeV electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall, these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis.

Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations

AbstractThe dual‐spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth's radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months‐long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi‐MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both “competing” mechanisms of multi‐MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.

Characterizing the Earth's outer Van Allen zone using a radiation belt content index

We have utilized knowledge acquired from near‐Earth spacecraft missions to construct a “radiation belt content” (RBC) index. This index sums radiation belt electron fluxes above several chosen threshold energies (most commonly, E > 2 MeV) and over the range of magnetic L shells 2.5 ≤ L ≤ 6.5. The method takes account of pitch angle distribution effects and magnetic flux tube properties. As constructed, the RBC index gives a simple, robust, and readily utilized daily estimate of the total number of electrons throughout the entire outer Van Allen radiation belt. We show correlative comparisons with concurrent solar wind parameters, geomagnetic indices, and geostationary orbit (GOES) data sets. We also examine statistical and recurrence characteristics of the RBC index itself. Results are compared with some of the orbit‐integrated dose results from the Highly Elliptical Orbit (HEO) satellite 1997‐068 that covers the same L range. The correlation of the RBC index with various spacecraft data clearly illustrates the remarkable coherence of the outer Van Allen radiation belt previously described, for example, by Kanekal et al. [2001]. Although the RBC index may be viewed as suppressing much of the spatial information content of the direct electron measurements in the outer trapping zone, it nonetheless gives a useful tool for statistical and time series analysis. The utilization of such a long‐term, homogeneous index for space weather purposes is suggested.

The areas of motion in the three dipole problem

In this article we give for the first time the qualitive characteristics of the areas of motion for a charged particle moving under the influence of the electromagnetic field of three dipoles located on a system of three celestial bodies. Using the steepest descent method we define the zero velocity curves of these areas for a variety of values of the electromagnetic parameters of the problem. So we reveal the trapped areas of motion of the charged particle called Van-Allen zones. As the parameters of this problem tend to zero the Stormer's and Magnetic Binary problems appeared in a general form.

New types of motion in St�rmer's problem

Application of boundary value techniques in the case of an electron performing relativistic motions within a magnetic dipole such as that of the Earth supplemented by a scanning process by means of which the entire phase space of the problem can be investigated, six new types of periodic motion have been discovered and computed. The stability of these motions is investigated and their direct bearing on formation and shape of the Van Allen zones of the Earth is discussed.

Differential energy spectrum of geomagnetically trapped protons with the Esro 2 satellite

The proton energy spectrum in the inner Van Allen zone was investigated in the range 30 to 300 Mev with a solid state dE/dx telescope aboard the Esro 2 satellite. The B-L domain covered is 1.3 ≤ L ≤ 2.4 and 0.18 ≤ B ≤ 0.24. It was found that an exponential spectrum f [E] = N0 exp (—E/E0) is a good fit to the data in the range L = 2 to L = 2.4. For L < 2, a composite exponential spectrum is more suitable for describing the observed shape: Between 30 Mev and a ‘knee-energy’ Ec, f[E] = N1 exp (—E/E1) and for E > Ec, f[E] = N0 exp (—E/E0). From our data, the variations with L of the parameters Ec, E0, E1 were found to be (a) E1 = (148 ± 8) Mev; (b) E0 = (290 ± 20) L−3.3±0.3; and (c) Ec = (400 ± 20) L−3±0.2. We suggest that two injection mechanisms are operating for all of this energy range, and that they give rise to two populations. The first one is the main component at energies lower than 25 Mev. At energies higher than our threshold (30 Mev), the detected fluxes are the addition of a second population (main component) and the high-energy tail of the first population. The spectrum of this second population appears to have the characteristic shape of a spectrum altered by ionization losses and generated by a continuous source, the slope for energies greater than Ec being E0.

Some observations on energetic electrons in the Outer Van Allen Zone during auroral substorms in relation to open and closed field lines

Some observations on energetic electrons in the Outer Van Allen Zone during auroral substorms in relation to open and closed field lines

Consequences of a magnetospheric plasma

We explore the hypothesis that, in some cases, instabilities in the magnetospheric plasma create the maximum dissipation available to them. In the case of pitch‐angle diffusion instabilities, the tendency toward isotropy on auroral lines of force is evidence for strong pitch‐angle diffusion. The location of the auroral precipitation boundary may be computed by balancing convection with strong‐diffusion precipitation. The radial diffusion required to balance precipitation losses of energetic protons and electrons in the Van Allen zone could approach the Bohm diffusion rate, probably another maximum dissipation limit, at least for instabilities in the drift scaling.

Electrons in the Van Allen Zone measured with a scintillator on Explorer 14

The energy spectrum of electrons in the energy range from 5 to 160 kev is investigated near the equator, at L shells between 2 and 10, by using scintillator measurements on Explorer 14. We find: (1) peak values of 8×108 electrons/cm2/sec with an average energy of ∼50 kev at L∼5; (2) peak values of 1.6×109 electrons/cm2/sec with an average energy of ∼10 kev at L≳8; (3) time variations of intensity by factors of 10 or less above L∼4 during the 15 days of data under study; and (4) a discontinuity of the energy spectrum at L∼7, with a general tendency for the spectrum to become softer at higher L. The 10-kev electrons measured at L∼8 are trapped in the earth's magnetic field and appear not to be a tail of the outer belt electrons. These electrons are unable, without an accompanying replenishing mechanism, to supply the auroral regions.

Trapped protons in the South Atlantic magnetic anomaly, July through December 1961: 3. Magnetic storms and solar proton events

With Injun 1 data for the inner Van Allen zone in the South Atlantic anomaly, it is shown that trapped protons 1 Mev 40 Mev were not affected by the solar proton events and magnetic storms of July–December 1961. In the area near Sao Jose dos Campos, Brazil, for July 14–28, a period of intense and sustained activity with four solar proton events and six magnetic storms, the gross characteristics of the proton fluxes were similar to those for the surrounding quiet periods of July 1–12 and August 2–28: for protons >40 Mev the deviation from the local mean of the total period averages ±19%; for the 1- to 15-Mev protons, ±29%. For the region near Salisbury, Southern Rhodesia, the data were examined for intervals within the August–December period, which contained one major and four minor solar-proton events and eleven storms. Here, in their gross characteristics, protons >40 Mev had an average stability of ±30% and 1- to 15-Mev protons, ±31%. In a detailed examination for protons >40 Mev, a stability to within an average of ±7% was found to exist at seventeen points in B-L space, each point being sampled more than once by the satellite during the July–December period. In addition, these (high energy) particles had an unchanging flux in the slot (2.1 < L < 2.5) marking for these protons the outer boundary of the inner Van Allen zone, a region where the intensities are consistently low and, hence, where changes can be readily detected. The study of the inner zone behavior during the solar proton magnetic storm event of September 28 to October 4 yields results at least suggestive of an unchanging inner zone character during the event itself.

Radioactivity induced in sodium iodide by trapped protons

The earth satellite Oso 1 contained as one of its prime instruments the University of Minnesota γ-ray experiment, which was designed to search for extraterrestrial γ rays in the 50-kev to 3-Mev range [Peterson and Howard, 1961]. The detection system consisted of an array of NaI scintillation counters arranged in various logic and shielding configurations to provide directional properties and to reject unwanted cosmic-ray-produced background. In this letter we report on radioactivity induced in the NaI crystals during the brief periods when the satellite passed through the lower edge of the inner Van Allen zone over the South Atlantic region. The principal detector with an isotropic response was a phoswich scintillation counter [Peterson and Nitardy, 1961] which consisted of a 5.1-cm diameter by 5.4-cm long NaI crystal surrounded by a 0.32-cm layer of plastic phosphor. The assembly was viewed by a single photomultiplier. Energy losses greater than about 0.5 Mev in the plastic generated an anticoincidence gate, thus rejecting particle-like events. Gamma-ray-like events were divided into two energy ranges, 0.3 to 1.0 Mev and 1.0 to 3.0 Mev, and were accumulated continuously in binary registers which were read out about once every 70 seconds. The counter was surrounded by about 5 g/cm2 of aluminum, magnesium, and electronics and was located about 18 inches from the center of gravity of a 450-pound payload configuration.

Atmospheric neutron flux measurements

The energetic neutron flux at atmospheric depths from 160 to 300 g/cm−2 has been measured over latitudes extending from the equator to 55°N geomagnetic. Comparison of data with previous measurements indicates that at the time of these measurements the neutron spectrum was steeper than that determined by Hess et al. for neutrons of E > 60 Mev but was of the same form for E 60 Mev was detected on July 21, 1962, over a region from 50° to 55°N geomagnetic. The increase is possibly attributable to a dumping of protons temporarily trapped in the slot between the inner and outer Van Allen zones.

On the inner Van Allen zone of the earth

A description of the inner Van Allen zone in terms of Mcllwain’s co-ordinates is given. Exponential spectra for electrons and protons are shown to be the most suitable for expressing the results of various experiments. At L=1.2 and at the geomagnetic equator there are 104 exp [—E/160 keV] electrons/(cm2 s sr keV) perpendicular to the field lines, and 23 exp [— E/120 MeV] protons/(cm2 s MeV). The time variations detected by a Geiger counter in Explorer VII, at the magnetic shell with L>1.5 are ascribed to electrons. For ≤1.5 a fractional increase of 2.3 occurs during an 11 months time interval but the inner belt geometry is shown to remain unchanged with time. Therefore it is suggested that the same source which generates the trapped particles is also responsible for the time variation.

On the energy spectrum of protons trapped in the Earth's Inner Van Allen Zone

Determination of the proton energy spectrum in the inner van allen zone using data obtained from geiger counters on the explorer iv

Motions of charged particles trapped in the Earth's magnetosphere

Recent measurements by earth satellites provide conclusive evidence that the magnetosphere of the earth is distorted by the solar wind, as has been predicted on theoretical grounds for some time. In the work reported here motions of charged particles trapped in the distorted magnetosphere were computed by using, as a model of the magnetosphere, the field of a weak magnetic dipole in the presence of a strong magnetic dipole. Drift paths through this field were calculated both for particles whose motion is confined to the magnetic equatorial plane and for particles mirroring at low altitudes. The paths of particles of various energies moving in the magnetic equatorial plane under the combined influences of field gradient-induced drift and field rotation were also calculated. It was found that, in the model used, the field rotation energizes trapped particles on the morning side and de-energizes them on the evening side of the earth. It is suggested that this effect must occur in any realistic model of the distorted magnetosphere and that it may play a role in the accumulation of the energetic particles that constitute the Van Allen zones.

Detection of an intense flux of low-energy protons or ions trapped in the inner radiation zone

Employing a set of CdS crystal energy-flux detectors flown in satellite Injun I, we have observed an energy flux in excess of 50 ergs/cm2 sec sterad at approximately 1000 km altitude in the inner Van Allen zone. Through discrimination by magnetic rigidity and with the help of data from other on-board detectors, it was found that the flux is comprised of protons of energy 0.5 kev ≲E≲1 Mev and/or other heavy ions, apparently trapped in the geomagnetic field. So large a flux of low-energy heavy particles seems to be markedly inconsistent with prevailing ideas of the source function and/or the trapped lifetime of such particles.

Charged-particle variations in the outer Van Allen zone during a geomagnetic storm

Comments and data are preserted on the phenomena observed by instruments on the Explorer VI satellite. The data associated with the intense and unusual fluctuations in the particle intensity of the August 16, 1959, geomagnetic storm were compared with the data obtained by the spin coil magnetometer. This comparison revealed that a 180 deg out-of-phase correlation existed between the particle intensity and the magnetic field strength. The storm was accompanied by an increase in the scintillation count rate in the outer zone within 12 hr following the sudden commencement of the storm, whereas the instruments sensitive to bremsstrahlung from electrons showed a decrease. These observations are consistent with the geomagnetic storm model of Dessler and Parker, and the confirmation of the increase of energetic protons in the geomagnetic field during a storm makes plausible the establishment of the field stresses which accourt for the main phase of a geomagnetic storm in that model. (B.O.G.)

Geomagnetic storm theory

A discussion of the two-gas theory of the transmission of geomagnetic disturbances through the atmosphere (to several earth radii) is extended, with the following results: (i) The central problem concerning the main phase of a geomagnetic storm is the mechanism of penetration of solar ions into the geomagnetic field. An explanation is given depending on a combination of a uniform electric space-charge field and a system of irregular fields. (ii) A model of the main phase of a geomagnetic storm is given, the principal feature of which is a ‘magnetic tail’ extending from the earth on the dark side. (iii) The model may help to explain some other effects: the Gegenschein, electrons with auroral energies, the location of the Van Allen zones, and diurnal cosmic-ray variations, (iv) All observed geomagnetic disturbances have their sources initially in current systems in the lower ionosphere. Some are subsequently maintained by current systems in the earth itself and in the region of interaction between the solar and terrestrial plasmas. Others, mainly polar and equatorial, are maintained by ionospheric currents driven by space-charge electric fields, (v) Any ring current outside the geomagnetic field could cause an increase in the horizontal component. A westward-flowing ring current embedded in the field could cause either an increase or a decrease in the horizontal component. The basic effect is not the current but a sustained inward or outward mechanical force on the material in which the current flows.