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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
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NASA NTRS1983

Solar cycle variations in the interplanetary magnetic field

ISEE 3 interplanetary magnetic field measurements have been used to extend the NSSDC hourly averaged IMF composite data set through mid-1982. Most of sunspot cycle 20 (start:1964) and the first half of cycle 21 (start:1976) are now covered. The average magnitude of the field was relatively constant over cycle 20 with approx. 5-10% decreases in 1969 and 1971, when the Sun's polar regions changed polarity, and a 20% decrease in 1975-6 around solar minimum. Since the start of the new cycle, the total field strength has risen with the mean for the first third of 1982 being about 40% greater than the cycle 20 average. As during the previous cycle, an approx. 10% drop in IMF magnitude accompanied the 1980 reversal of the solar magnetic field. While the interplanetary magnetic field is clearly stronger during the present solar cycle, another 5-7 years of observations will be needed to determine if cycle 21 exhibits the same modest variations as the last cycle. Accordingly, it appears at this time that intercycle changes in IMF magnitude may be much larger than the intracycle variations.

Slavin, J. A.↗
NASA NTRS1978

Using dynamo theory to predict the sunspot number during solar cycle 21

On physical grounds it is suggested that the polar field strength of the sun near a solar minimum is closely related to the solar activity of the following cycle. Four methods of estimating the polar magnetic field strength of the sun near solar minimum are employed to provide an estimate of the yearly mean sunspot number of cycle 21 at solar maximum of 140 + or - 20. This estimate may be considered a first-order attempt to predict the cycle activity using one parameter of physical importance based upon dynamo theory.

Schatten, K. H.↗
NASA NTRS2016

Unusual Polar Conditions in Solar Cycle 24 and Their Implications for Cycle 25

We report on the prolonged solar-maximum conditions until late 2015 at the north-polar region of the Sun indicated by the occurrence of high-latitude prominence eruptions (PEs) and microwave brightness temperature close to the quiet-Sun level. These two aspects of solar activity indicate that the polarity reversal was completed by mid-2014 in the south and late 2015 in the north. The microwave brightness in the south-polar region has increased to a level exceeding the level of the Cycle 23/24 minimum, but just started to increase in the north. The northsouth asymmetry in the polarity reversal has switched from that in Cycle 23. These observations lead us to the hypothesis that the onset of Cycle 25 in the northern hemisphere is likely to be delayed with respect to that in the southern hemisphere. We find that the unusual condition in the north is a direct consequence of the arrival of poleward surges of opposite polarity from the active region belt. We also find that multiple rush-to-the-pole episodes were indicated by the PE locations that lined up at the boundary between opposite-polarity surges. The high-latitude PEs occurred in the boundary between the incumbent polar flux and the insurgent flux of opposite polarity.

Gopalswamy, Nat↗
NASA NTRS2014

Major Solar Eruptions and High-Energy Particle Events During Solar Cycle 24

We report on a study of all major solar eruptions that occurred on the frontside of the Sun during the rise to peak phase of cycle 24 (first 62 months) in order to understand the key factors affecting the occurrence of large solar energetic particle events (SEPs) and ground level enhancement (GLE) events. The eruptions involve major flares with soft X-ray peak flux greater than or equal to 5.0 x10(exp−5) Wm(exp−2) (i.e., flare size greater than or equal to M5.0) and accompanying coronal mass ejections (CMEs). The selection criterion was based on the fact that the only front-side GLE in cycle 24 (GLE 71) had a flare size of M5.1. Only approximately 37% of the major eruptions from the western hemisphere resulted in large SEP events. Almost the same number of large SEP events was produced in weaker eruptions (flare size less than M5.0), suggesting that the soft X-ray flare is not a good indicator of SEP or GLE events. On the other hand, the CME speed is a good indicator of SEP and GLE events because it is consistently high supporting the shock acceleration mechanism. We found the CME speed, magnetic connectivity to Earth (in longitude and latitude), and ambient conditions as the main factors that contribute to the lack of high-energy particle events during cycle 24. Several eruptions poorly connected to Earth (eastern-hemisphere or behind-the-west-limb events) resulted in very large SEP events detected by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. Some very fast CMEs, likely to have accelerated particles to GeV energies, did not result in a GLE event because of poor latitudinal connectivity. The stringent latitudinal requirement suggests that the highest-energy particles are likely accelerated in the nose part of shocks, while the lower energy particles are accelerated at all parts. There were also well-connected fast CMEs, which did not seem to have accelerated high-energy particles due to possible unfavorable ambient conditions (high Alfven speed, overall reduction in acceleration efficiency in cycle 24).

Flares↗
NASA NTRS1995

Regions of abnormally low proton temperature as signatures of ejecta: Solar cycle dependence and association with other ejecta signatures

Solar wind proton temperatures lower than expected for 'normal' solar wind expansion are a common signature of 'ejecta' (i.e. interplanetary coronal mass ejections). We have surveyed the OMNI solar wind data base for 1965-1991, and Helios data for 1974-1980, to identify regions of abnormally low temperatures. Their occurrence rate is clearly dependent on solar activity levels, in particular when the minority of events associated with encounters with the heliospheric plasma sheet are excluded. The analysis of the OMNI data may provide an indication of the rate of ejecta at the Earth, and hence of the CME rate, extending back to before spacecraft coronagraph observations became available in the early 1970's. We discuss the association of these solar wind structures with cosmic ray depressions bidirectional particle flows, and other ejecta signatures. Our impression is that no one ejecta signature provides a truly comprehensive indication of the presence of ejecta, but that abnormally low temperature depressions encompass most of the regions identified by these other individual signatures.

Richardson, I. G.↗
NASA NTRS1992

Counterstreaming solar wind halo electron events - Solar cycle variations

It is shown that during the interval from August 1978 through December 1990 the frequency of the occurrence of counterstreaming events varied roughly in phase and amplitude with coronal mass ejecta (CME) rates derived from coronagraph observations and with the advance of the solar activity cycle as measured, e.g., by sunspot numbers. It is inferred that CMEs in the solar wind near 1 AU can usually be identified by the counterstreaming solar wind halo electron signature.

Gosling, J. T.↗
NASA NTRS1991

The solar cycle variation of the rates of CMEs and related activity

Coronal mass ejections (CMEs) are an important aspect of the physics of the corona and heliosphere. This paper presents results of a study of occurrence frequencies of CMEs and related activity tracers over more than a complete solar activity cycle. To properly estimate occurrence rates, observed CME rates must be corrected for instrument duty cycles, detection efficiencies away from the skyplane, mass detection thresholds, and geometrical considerations. These corrections are evaluated using CME data from 1976-1989 obtained with the Skylab, SMM and SOLWIND coronagraphs and the Helios-2 photometers. The major results are: (1) the occurrence rate of CMEs tends to track the activity cycle in both amplitude and phase; (2) the corrected rates from different instruments are reasonably consistent; and (3) over the long term, no one class of solar activity tracer is better correlated with CME rate than any other (with the possible exception of type II bursts).

Webb, David F.↗
NASA NTRS1995

Flow downstream of the heliospheric terminal shock: Magnetic field line topology and solar cycle imprint

The topology of the magnetic field in the heliosheath is illustrated using plots of the field lines. It is shown that the Archimedean spiral inside the terminal shock is rotated back in the heliosheath into nested spirals that are advected in the direction of the interstellar wind. The 22-year solar magnetic cycle is imprinted onto these field lines in the form of unipolar magnetic envelopes surrounded by volumes of strongly mixed polarity. Each envelope is defined by the changing tilt of the heliospheric current sheet, which is in turn defined by the boundary of unipolar high-latitude regions on the Sun that shrink to the pole at solar maximum and expand to the equator at solar minimum. The detailed shape of the envelopes is regulated by the solar wind velocity structure in the heliosheath.

Nerney, Steven↗
NASA NTRS1993

On the latitude and solar cycle dependence of the interplanetary magnetic field strength

Twenty years of solar magnetograph and in-ecliptic IMF measurements are used to verify that the radial IMF component (B sub r) can be approximated by a superposition of the sun's inclined magnetic dipole moment and a current sheet normal to the dipole axis. The net field is found to be about three times stronger along the dipole axis than near the current sheet. Over the sunspot cycle, the absolute value of B sub r should undergo the least modulation near the heliographic equator and the greatest modulation above the sun's poles, where, at 1 AU, it varies systematically between about 10 nT near sunspot minimum and about 1 nT near sunspot maximum. The latitudinal gradients in the absolute value of B sub r are expected to be steepest around sunspot minimum and flattest near sunspot maximum. It is predicted that Ulysses will encounter very strong fields when it flies over the solar poles during the declining phase of sunspot cycle 22.

Wang, Y.-M.↗
NASA NTRS1977

Solar wind, energetic particles, and coronal magnetic structure - The first year of solar cycle 20

Coronal magnetic field structure is investigated by using solar and interplanetary magnetic polarity measurements and by using observations of solar wind plasma and energetic particles during the first 8 months of 1965. When it is compared by using all data available during this period, interplanetary polarity (mapped back to the high corona by using observed solar wind velocities) correlates best with mid-latitude solar polarity. However, when it is compared at only those times when energetic particles were present in the interplanetary medium and again at times of fast solar wind, the best correlation is with equatorial solar polarity. From these results we infer that energetic particles and fast solar wind escape preferentially from equatorial coronal magnetic structures which are open, while the more usual equatorial structures at this time are closed.

Nolte, J. T.↗
NASA NTRS2023

Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

Launched on 12 Aug. 2018, NASA’s Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission’s primary science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-Alfvénic solar wind, which is one of the mission’s primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles.

Sun↗