71 research outputs found
A BATSE investigation of radiation belt electrons precipitated by VLF waves
The Compton Observatory commonly encounters fluxes of energetic electrons which have been scattered from the inner radiation belt to the path of the satellite by resonant interactions with VLF waves from powerful man-made transmitters. The present investigation was motivated by the fact that in the Fall of 1993, the Gamma Ray Observatory was boosted from a 650 km altitude circular orbit to a 750 km altitude circular orbit. This was an opportunity, for the first time, to make observations at two different altitudes using the same instrument. We have examined DISCLA data from the Burst & Transient Source Experiment (BATSE) experiment from 1 Sep. 1993 to 29 Jan. 1994. During the period of study we identified 48 instances of the satellite encountering a cloud of energetic electrons which had been scattered by VLF transmitters. We find that boosting the altitude of the circular orbit from 650 km to 750 km increased the intensity of cyclotron resonance scattered electrons by a factor of two. To search for long term changes in the cyclotron resonance precipitation, we have compared the approx. 750 km altitude data from 106 days at the end of 1993 with data at the same altitudes and time of year in 1991. The cyclotron resonance events in 1991 were three times more frequent and 25% of those cases were more intense than any seen in the 1993 data. We attribute this difference to increased level of geomagnetic activity in 1991 near the Solar Maximum
X-ray bursts from solar flares behind the limb
X-ray bursts are identified from the UCSD OSO-7 X-ray experiment data. X-ray spectroheliograms of OSO-5, H alpha activity at the limb, and the emergence and disappearance of sunspot groups at the limb were studied and 17 active centers were found as likely seats of the X-ray bursts beyond the limb. The analysis of 37 X-ray bursts and their physical parameters is presented. Results show that (1) the distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events; (2) that radiation is the dominant cooling mechanism for the hot flare plasma; and (3) that the scale height for X-ray emission in the 5-10 keV range is large. Observations show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Hard X-ray emission over extended regions is indicated
A BATSE investigation of radiation belt electrons precipitated by VLF waves
The Compton Observatory commonly encounters fluxes of energetic electrons which have been scattered from the inner radiation belt to the path of the satellite by resonant interactions with VLF waves from powerful man-made transmitters. The present investigation was motivated by the fact that in the Fall of 1993, the Gamma Ray Observatory was boosted from a 650 km altitude circular orbit to a 750 km altitude circular orbit. This was an opportunity, for the first time, to make observations at two different altitudes using the same instrument. We have examined DISCLA data from the Burst & Transient Source Experiment (BATSE) experiment from 1 Sep. 1993 to 29 Jan. 1994. During the period of study we identified 48 instances of the satellite encountering a cloud of energetic electrons which had been scattered by VLF transmitters. We find that boosting the altitude of the circular orbit from 650 km to 750 km increased the intensity of cyclotron resonance scattered electrons by a factor of two. To search for long term changes in the cyclotron resonance precipitation, we have compared the approx. 750 km altitude data from 106 days at the end of 1993 with data at the same altitudes and time of year in 1991. The cyclotron resonance events in 1991 were three times more frequent and 25% of those cases were more intense than any seen in the 1993 data. We attribute this difference to increased level of geomagnetic activity in 1991 near the Solar Maximum
Compton Gamma Ray Observatory/BATSE observations of energetic electrons scattered by cyclotron resonance with waves from powerful VLF transmitters
To obtain a better understanding of the wave-particle mechanisms responsible for the loss of electrons from the radiation belts, energetic electron data from the Burst and Transient Source Experiment (BATSE) on the NASA's Compton Gamma Ray Observatory (GRO) was studied. Powerful ground-based VLF transmitters resonantly scatter electrons from the inner radiation belt onto trajectories from which they precipitate into the atmosphere as they drift eastward. 563 instances in which the satellite traversed a cloud of energetic electrons which had been scattered into quasi-trapped trajectories were identified. From the longitude distribution, it was concluded that waves from the VLF transmitter NWC at 114 deg E are the origin of 257 of the events, and waves from UMSat 44 deg E related to 45 more. In another 177 cases the electrons had drifted from the longitude of these transmitters to a location in the western hemisphere. The previously reported seasonal variation in the frequency of occurrence of cyclotron resonance interaction is confirmed with the continuous coverage provided by GRO. The frequency of occurrence of the cyclotron resonance interactions is largest before sunrise, which we attribute to the diurnal variations in the transmission VLF waves through the ionosphere. For the first time, unique very narrow sheets of electrons occurring in the aftermath of a large geomagnetic storm are reported
The Lockheed OSO-8 program. Analysis of data from the mapping X-ray heliometer experiment
The final report describes the extent of the analysis effort, and other activities associated with the preservation and documentation of the data set are described. The main scientific results, which are related to the behavior of individual solar activity regions in the energy band 1.5 - 15 keV, are summarized, and a complete bibliography of publications and presentations is given. Copies of key articles are also provided
Characterising a Si(Li) detector element for the SIXA X-ray spectrometer
The detection efficiency and response function of a Si(Li) detector element
for the SIXA spectrometer have been determined in the 500 eV to 5 keV energy
range using synchrotron radiation emitted at a bending magnet of the electron
storage ring BESSY, which is a primary radiation standard. The agreement
between the measured spectrum and the model calculation is better than 2%.
PACS: 95.55.Ka; 07.85.Nc; 29.40.Wk; 85.30.De
Keywords: Si(Li) detectors, X-ray spectrometers, detector calibration, X-ray
response, spectral lineshapeComment: 11 pages, 11 PostScript figures, uses elsart.sty, submitted to Nucl.
Instrum. Meth.
A nanoflare model of quiet Sun EUV emission
Nanoflares have been proposed as the main source of heating of the solar
corona. However, detecting them directly has so far proved elusive, and
extrapolating to them from the properties of larger brightenings gives
unreliable estimates of the power-law exponent characterising their
distribution. Here we take the approach of statistically modelling light curves
representative of the quiet Sun as seen in EUV radiation. The basic assumption
is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose
radiative energies display a power-law distribution. Radiance values in the
quiet Sun follow a lognormal distribution. This is irrespective of whether the
distribution is made over a spatial scan or over a time series. We show that
these distributions can be reproduced by our simple model.Comment: 13 pages, 18 figures, accepted for publication by A&
The State of Self-Organized Criticality of the Sun During the Last 3 Solar Cycles. I. Observations
We analyze the occurrence frequency distributions of peak fluxes , total
fluxes , and durations of solar flares over the last three solar cycles
(during 1980--2010) from hard X-ray data of HXRBS/SMM, BATSE/CGRO, and RHESSI.
From the synthesized data we find powerlaw slopes with mean values of
for the peak flux, for the total
flux, and for flare durations. We find a systematic
anti-correlation of the powerlaw slope of peak fluxes as a function of the
solar cycle, varying with an approximate sinusoidal variation
, with a
mean of , a variation of , a solar cycle
period yrs, and a cycle minimum time . The
powerlaw slope is flattest during the maximum of a solar cycle, which indicates
a higher magnetic complexity of the solar corona that leads to an
overproportional rate of powerful flares.Comment: subm. to Solar Physic
Simulating Flaring Events in Complex Active Regions Driven by Observed Magnetograms
We interpret solar flares as events originating from active regions that have
reached the Self Organized Critical state, by using a refined Cellular
Automaton model with initial conditions derived from observations. Aims: We
investigate whether the system, with its imposed physical elements,reaches a
Self Organized Critical state and whether well-known statistical properties of
flares, such as scaling laws observed in the distribution functions of
characteristic parameters, are reproduced after this state has been reached.
Results: Our results show that Self Organized Criticality is indeed reached
when applying specific loading and relaxation rules. Power law indices obtained
from the distribution functions of the modeled flaring events are in good
agreement with observations. Single power laws (peak and total flare energy) as
well as power laws with exponential cutoff and double power laws (flare
duration) are obtained. The results are also compared with observational X-ray
data from GOES satellite for our active-region sample. Conclusions: We conclude
that well-known statistical properties of flares are reproduced after the
system has reached Self Organized Criticality. A significant enhancement of our
refined Cellular Automaton model is that it commences the simulation from
observed vector magnetograms, thus facilitating energy calculation in physical
units. The model described in this study remains consistent with fundamental
physical requirements, and imposes physically meaningful driving and
redistribution rules.Comment: 14 pages; 12 figures; 6 tables - A&A, in pres
Statistical characterisation of full-disk EUV/XUV solar irradiance and correlation with solar activity
We investigate the distribution of fluctuations in solar irradiance when
integrated over the full disk, obtained using extreme ultraviolet/soft X-ray
observations from the SOHO CELIAS/SEM instrument. This time series sums over
both the contributions of single distinguishable flares, and of many other
processes. By detrending we select events with timescales of less than a few
hours such as waves, slow flows, and CMEs. The statistics generated by this
range of phenomena can be characterised by power-law-tailed distributions. We
show that (i) during the high-activity period 2000 Jan-June the tail exponent
a(T)=1.5 +/- 0.1; (ii) during the low-activity period 1996 Jan-June a(T)=3.0
+/- 0.2; and (iii) in general a(T) decreases with increasing activity.Comment: 4 pages, 5 figures; v.2 R-squared goodness of fits adde
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