Value chain · CLM-0154
In plain terms
A 2004 study using seven years of real radio-wave-absorption measurements from northern Finland found a rough mathematical relationship between geomagnetic storm strength and radio absorption, but the relationship only weakly predicts what actually happens on any given day.
The claim as recorded
Kavanagh et al. (2004), using seven years (1995-2001) of measured Imaging Riometer (IRIS, Kilpisjarvi, Finland, 38.2 MHz) data, fit a quadratic relation Am = a x Kp^2 + b x Kp + c between auroral absorption and the Kp geomagnetic index for both the mean and the median absorption; in the steepest sector (09:00-12:00 MLT) the median fit (a=0.035, b=-0.026, c=0.070) gives about 0.31 dB at Kp=3 and 2.67 dB at Kp=9, and the mean fit (a=0.040, b=-0.028, c=0.086) about 0.36 and 3.07 dB. The paper reports a correlation of r=0.51 for this fit against the full dataset (r=0.54 restricted to well-populated Kp bins), and says that high absorption is possible at low Kp and low absorption is common even at high Kp.
Scope and limits
A HIGH-LATITUDE (auroral-zone, ~66N magnetic) measured climatology at HF (38.2 MHz), not at a sub-auroral latitude (such as the UK's ~54N magnetic) or at microwave/SPS frequency (2.45 GHz); transferring it to either a lower latitude or a higher frequency requires additional, unverified scaling assumptions (see CLM-0155). The stated correlation (r=0.51) is modest even within its own dataset; the paper itself calls it notably higher than its earlier logarithmic fit's, so "weak" is this register's reading, carried by the paper's own "poor predictors" sentence.
What it assumes
The Kilpisjarvi riometer site (69.05N geographic, ~66N magnetic) is representative of general auroral-zone absorption behaviour; a single-station climatology, not a global one.
Where it comes from
Kavanagh et al. (2004), Ann. Geophys. 22, printed p.882 (PDF p.6), Eq. (2), whose constants are listed in Table 3 "for both the mean and median absorption"; Table 3 on printed p.883 (PDF p.7), 09:00-12:00 MLT row: mean 0.040, -0.028, 0.086; median 0.035, -0.026, 0.070; correlation "r=0.51 (r=0.54 restricted to well-populated Kp bins)"; "the variability of absorption in the Kp bins suggests that the statistical mean and median are poor predictors of auroral absorption" (p.882). Absorption values at Kp=3 and 9 are this register's arithmetic from the coefficients.
Basis: absorption in dB at 38.2 MHz, per unit Kp, zenith beam, 10-minute averages
Related records
- Answers question
- QST-001 not published in this release
Cite this
energyaperture.com/c/clm-0154
CLM-0154, Energy Aperture, as of 2004 (data: 1995-2001).