Geology topics

J.L. Gannon

Publications and source records attributed to J.L. Gannon.

5 recordsLinked to original sources

USGS2012

Problem of the Love‐Gannon relation between the asymmetric disturbance field and Dst

Love and Gannon (2009) discovered that statistically, over a fifty year period the difference in the dawn and dusk disturbance‐field H component at low latitudes (hourly averaged) is linearly proportional to Dst. If the difference is designated by δ DD in units of nT/R E , then the Love‐Gannon (L‐G) relation is δ DD = − 0.2 Dst. At any time departures from the relation can be large. Nonetheless, the relation is evident for all values of Dst and persists throughout magnetic storms, both the main phase and the recovery phase. The Love‐Gannon discovery presents a problem to current understanding of the relation between the causes of δ DD and Dst because the dawn dusk asymmetry in the disturbance field is presumably governed by a long‐established magnetosphere‐ionosphere coupling theory which predicts a characteristic time scale (the shielding time) of less than an hour whereas the characteristic time scale for Dst (the ring current decay time) is more like ten hours. Thus, without forcing both time scales toward each other to the limits of their ranges, a linear proportionality between δ DD and Dst cannot be derived from the current understanding of the causes of the asymmetry and the ring current. This conclusion is the paper's main contribution. In addition, we attempt to get around the conflict of time scales by looking at other possibilities for generating δ DD that depend directly on the ring current. The most promising of these is the possibility that the ring current decay mechanism creates a quasi‐permanent, local‐time modification of the ring current compared to what it would be in the absence of the decay mechanism and that this modification causes a field‐aligned current that closes through the ionosphere and generates the asymmetry δ DD . This idea has the virtue of coupling the asymmetry directly to the ring current and of accounting for the persistence of the L‐G proportionality through the recovery phase of magnetic storms.

Journal of Geophysical Research A: Space Physics↗

USGS2012

Superposed epoch analysis and storm statistics from 25 years of the global geomagnetic disturbance index, USGS-Dst

Statistics on geomagnetic storms with minima below -50 nanoTesla are compiled using a 25-year span of the 1-minute resolution disturbance index, U.S. Geological Survey Dst. A sudden commencement, main phase minimum, and time between the two has a magnitude of 35 nanoTesla, -100 nanoTesla, and 12 hours, respectively, at the 50th percentile level. The cumulative distribution functions for each of these features are presented. Correlation between sudden commencement magnitude and main phase magnitude is shown to be low. Small, medium, and large storm templates at the 33rd, 50th, and 90th percentile are presented and compared to real examples. In addition, the relative occurrence of rates of change in Dst are presented.

Open-File Report↗

USGS2011

USGS 1-min Dst index

W e produce a 1-min time resolution storm-time disturbance index, the USGS Dst, called Dst 8507-4 SM . This index is based on minute resolution horizontal magnetic field intensity from low-latitude observatories in Honolulu, Kakioka, San Juan and Hermanus, for the years 1985–2007. The method used to produce the index uses a combination of time- and frequency-domain techniques, which more clearly identifies and excises solar-quiet variation from the horizontal intensity time series of an individual station than the strictly time-domain method used in the Kyoto Dst index. The USGS 1-min Dst is compared against the Kyoto Dst, Kyoto Sym-H, and the USGS 1-h Dst ( Dst 5807-4 SH ). In a time series comparison, Sym-H is found to produce more extreme values during both sudden impulses and main phase maximum deviation, possibly due to the latitude of its contributing observatories. Both Kyoto indices are shown to have a peak in their distributions below zero, while the USGS indices have a peak near zero. The USGS 1-min Dst is shown to have the higher time resolution benefits of Sym-H, while using the more typical low-latitude observatories of Kyoto Dst.

Journal of Atmospheric and Solar-Terrestrial Physi↗

USGS2011

U.S. Geological Survey Near Real-Time Dst Index

The operational version of the United States Geological Survey one-minute Dst index (a global geomagnetic disturbance-intensity index for scientific studies and definition of space-weather effects) uses either four- or three-station input (including Honolulu, Hawaii; San Juan, Puerto Rico; Hermanus, South Africa; and Kakioka, Japan; or Honolulu, San Juan and Guam) and a method based on the U.S. Geological Survey definitive Dst index, in which Dst is more rigorously calculated. The method uses a combination of time-domain techniques and frequency-space filtering to produce the disturbance time series at an individual observatory. The operational output is compared to the U.S. Geological Survey one-minute Dst index (definitive version) and to the Kyoto (Japan) Final Dst to show that the U.S. Geological Survey operational output matches both definitive indices well.

Open-File Report↗

USGS2009

Revised Dst and the epicycles of magnetic disturbance: 1958-2007

A revised version of the storm-time disturbance index D st is calculated using hourly-mean magnetic-observatory data from four standard observatories and collected over the years 1958-2007. The calculation algorithm is a revision of that established by Sugiura et al., and which is now used by the Kyoto World Data Center for routine production of D st . The most important new development is for the removal of solar-quiet variation. This is done through time and frequency-domain band-stop filtering - selectively removing specific Fourier terms approximating stationary periodic variation driven by the Earth's rotation, the Moon's orbit, the Earth's orbit around the Sun, and their mutual coupling. The resulting non-stationary disturbance time series are weighted by observatory-site geomagnetic latitude and then averaged together across longitudes to give what we call D st 5807-4SH . Comparisons are made with the standard Kyoto D st . Various biases, especially for residual solar-quiet variation, are identified in the Kyoto D st , and occasional storm-time errors in the Kyoto D st are noted. Using D st 5807-4SH , storms are ranked for maximum storm-time intensity, and we show that storm-occurrence frequency follows a power-law distribution with an exponential cutoff. The epicycles of magnetic disturbance are explored: we (1) map low-latitude local-time disturbance asymmetry, (2) confirm the 27-day storm-recurrence phenomenon using autocorrelation, (3) investigate the coupled semi-annual-diurnal variation of magnetic activity and the proposed explanatory equinoctial and Russell-McPherron hypotheses, and (4) illustrate the well-known solar-cycle modulation of storm-occurrence likelihood. Since D st 5807-4SH is useful for a variety of space physics and solid-Earth applications, it is made freely available to the scientific community.

Annales Geophysicae↗