Table of Contents

• Planetary magnetic activity indices (K_p, a_p, and A_p)
• Planetary daily character figure (C_p, C9, C_i)
• Geomagnetic aa Index (aa, Kpa)
• Geomagnetic am Index (am, an, as, Km, Kn, Ks, Am, An, and As)
• Longitudinally asymmetric/symmetric disturbance index (Sym-H, Sym-D, Asy-H, Asy-D)
• Disturbance Storm-Time Index (D_st)
• Bartels solar rotation number
• International Sunspot number (R_z, R_i)
• Ottawa 10.7 cm Solar radio flux adjusted to 1 AU (F_10.7)
• Auroral Electrojet index (AE)
• Polar Cap Index (PC)
• Comprehensive Flare Index (CFI)
• Archives
• References

Planetary magnetic activity indices (K_p, a_p, and A_p)

The basic idea of using K-indices from a network of observatories to derive a planetary index of geomagnetic activity was proposed by Bartels et al. (1939) in the same paper in which the K indices were defined. The planetary 3-hour-range index K_p is the mean standardized K-index from 13 geomagnetic observatories (originally, there were 11 observatories involved, Lovö was added in 1954 and Canberra in 1970) between 45.2 degrees and 62.5 degrees northern or southern geomagnetic latitude.

The scale is O to 9 expressed in thirds of a unit, e.g. 5- is 4 2/3, 5o is 5 and 5+ is 5 1/3. This planetary index is designed to measure solar particle radiation by its magnetic effects. The 3-hourly a_p (equivalent range) index is derived from the K_p index as follows:

In order to use a_p as an equivalent amplitude, it is considered in relation to the conditions at a standard station, which is a station having the lower limit of 500 nT for K = 9. At such a station the average range in nT of the most disturbed of the two horizontal components in a three-hour interval can be taken as 2 a_p (for instance, for K_p = 3+, a_p = 18, i.e. 36 nT). In other words a_p is an equivalent amplitude in the unit 2 nT.

A_p gives the daily average for the eight values a_p per day. Therefore, it may be called the equivalent daily amplitude A_p, expressed in the unit 2 nT.

Since January 1997, K_p and derived indices are calculated at the GeoForschungsZentrum, Potsdam.

Planetary daily character figure (C_p)

The ranges for C_p were defined by making the frequency distributions of the older qualitative index C_i and C_p in the 10 years 1940 to 1949 practically the same. Since then, although obtained by entirely different definitions, C_i and C_p have rarely differed by more than 0.2.

The C_i index, the daily international character figure, was formed by taking the arithmetic mean of the individual C indices reported by all of the collaborating observatories. The C is a subjective index for a single observatory rating the magnetogram for the 24-h Greenwich day as 0 if very quiet, as 1 if moderately disturbed, or as 2 if severely disturbed.

The C9 index is a conversion of the C_p index to one digit between 0 and 9.

Geomagnetic aa index

For each three hour interval, K-indices measured at the two stations are converted back into amplitude. A three hour aa index is the mean of the northern and southern values, weighted to account for the small differences in the latitudes of the two stations, or for the slight changes in the very place of the observatory. The unit for the aa index is nT, and it represents the activity level at an invariant magnetic latitude of about 50 degrees. The daily values are formed from an average of the 8 three-hourly values, and are very close to the corresponding daily values of the am values.

The aa index comes from a request made by the Royal Society of London at the Madrid IAGA in 1969 by S. Chapman for C_i before 1884. Early records from Greenwich and Melbourne give a long series of data. Where early individual data records are missing other stations provide approximate information instead. The index is definitive from 1868 - 31 December 1996 and provisional from 1997.

Kpa is the code corresponding to the aa index, expressed in the same scale as K_p.

The index is calculated at the International Service Of Geomagnetic Indices.

Geomagnetic am index (am, an, as)

The am index was devised by Bartels and agreed by IAGA in Madrid in 1969. The am indices have been derived by several different groups.

The am indices are derived from observatory K index, corrected geomagnetic latitude, angular distance of observatory to the nearest point of the auroral zone, corrected geomagnetic Longitude of that point, computed lower limit for K = 9 and the actual limit used.

Two indices, an and as, are computed for the northern and southern hemispheres respectively, and the am index is the average of an and as.

Both daily and three-hourly data are available. The daily values Am, An and As are the daily mean values of the 8 three-hourly values am, an and as respectively.

For the sake of tradition and convenience, Km, Kn and Ks equivalent values are also made available by means of a conversion table; they are as usually expressed form 0o to 9o.

The am index records begin in 1959. Although provisional values are available during the year, even if all stations have not provided their data, final values are only published after the end of a year.

Longitudinally symmetric/asymmetric disturbance index (Sym-H, Sym-D, Asy-H, Asy-D)

Note that Sym-H is effectively the same as Sugiura's (1964) hourly D_st index, even though it uses 1-minute values from different sets of stations and a slightly different coordinate system. Also, Asy-H is close to the asymmetric indices proposed by Kawasaki and Akasofu (1971), Crooker and Siscoe (1971) and Clauer et al. (1983).

Finally, despite the fact that there is a rather good correlation between the Asy (Asy-H and Asy-D) and AE indices it should be noted that there are some fundamental differences between them (Iyemori et al., 1999). Thus, one should be careful when uses the Asy indices as a monitor of disturbances in the polar region.

Disturbance Storm-Time Index (D_st)

At a given time, the D_st index is the average of variation over all longitudes; the reference level is set so that D_st is statistically zero on internationally designated quiet days. An index of -50 nT or deeper indicates a storm-level disturbance, and an index of -200 nT or deeper is associated with middle-latitude auroras. The D_st index is described by Sugiura (1964).

D_st values correlate in gross features with the A_p indices. However, after the start of a geomagnetic storm, D_st recovers more slowly, indicating that the ring current dies away less rapidly than the polar disturbance.

Bartels solar rotation number

The Bartels solar rotation number was introduced by Bartels (1934) and adopted by IATME (1954) at Brussels. They are shorter than the Carrington rotations by 0.2753 days because they are made up of a whole number of days. The numbers, which form a sequence of 27-day intervals, are counted continuously from February 8, 1832.

International Sunspot number (R_z, R_i)

The provisional daily Zürich relative sunspot numbers, R_z, were based upon observations made at Zürich and its two branch stations in Arosa and Locarno and communicated by M. Waldmeier of the Swiss Federal Observatory. Beginning January 1, 1981, the Zürich relative sunspot number program is replaced by the "Sunspot Index Data Center" (c/o Dr. R. Van Der Lindenn, KSB-ORB, Ringlaan 3, B-1180 Brussels, Belgium, e-mail: Ronald.Vanderlinden@oma.be). The determination of the provisional International Sunspot Numbers R_i results from a statistical treatment of the data originating from more than twenty-five observing stations. These stations constitute an international network, with the Locarno (Switzerland) station as the reference station, to guarantee continuity with the past Zürich series of R_z. The definitive International Sunspot Numbers R_i are evaluated by a similar method based on a network of observing stations selected for their high number of observations, their continuity during the whole year and an existing series of observations during the last years. Also taken into account is the stability of the K monthly factors with reference to the Locarno station. These relative sunspot numbers are now designated R_i (International) instead of R_z (Zürich).

Since August 1992, the hemispheric sunspot numbers R_n and R_s are also provided. They are calculated the same way as the total sunspot numbers, separately for both hemispheres, the number of contributing stations being around 25 for the provisional values and 40 for the definitive ones. Locarno is also the reference station. The results are normalized to the International Sunspot Number, in order to satisfy the relation R_n + R_s = R_i.

The daily American relative sunspot numbers, R_a, are compiled by Peter O. Taylor (4523 Thurston Lane, #5, Madison, WI 53711 USA) for the Solar Division of the American Association of Variable Star Observers (AAVSO). The R_a observations are collected by an international network of extraordinarily faithful observers, many of them amateurs, and each with many years of experience. About 35 observers contribute to the preliminary R_a which is available on the 2nd of each month following the observed month. About 100 observers contribute to the final R_a values. The counts are made visually with a variety of suitably protected-telescopes.

Final values of R_i appear in SGD, in the IAU Quarterly Bulletin on Solar Activity, and elsewhere. They usually differ slightly from the provisional values. Final American numbers, R_a, are available by the 15th of the month following observation, and after collection of all observer reports.

The smoothed relative sunspot number is defined as:

in which R_k is the mean value of R for a single month k and R(12) is the smoothed index for the month represented by k = n.

The predicted sunspot numbers for the 12th month after the latest observation are computed using the method of A. G. McNish and J. V. Lincoln (1949) and modified using regression coefficients and mean cycle values computed for Cycles 8 through 20.

Ottawa 10.7 cm Solar radio flux adjusted to 1 AU (F_10.7)

The flux monitors have 1.8m paraboloidal antennas, which are equally sensitive to all points on the solar disc, and are equipped to measure emissions which are linearly-polarized in the North-South sense. In calculating the 10.7cm flux, it is assumed that the integrated emission from the solar disc at that wavelength has no net linear polarization. Fluxes are given in units of 10^-22 J s^-1 m^-2 Hz^-1, 10⁴ JANSKY or solar flux units (s.f.u.). Often used names for this index are F_10.7 and Covington index.The observed values are adjusted for the changing Sun-Earth distance (adjusted values) and for uncertainties in antenna gain (absolute values). The characteristics of the observations are surveyed in Covington (1969).

The data are tabulated in two forms: the "observed flux" (S), and the "adjusted flux" (Sa). The former are the actual measured values, and are affected by the changing distance between the Earth and Sun throughout the year, whereas the latter are scaled to a standard distance of 1 AU. The "observed flux" values are useful in ionospheric physics and other terrestrial consequences of solar activity. The "adjusted fluxes" are more purely descriptive of the Sun's behaviour.

Over long periods of time, the r.m.s. relative errors are not more than plus or minus 1% or one flux unit, whichever is larger. The absolute accuracy is a more complicated issue. Through extensive recalibrations and comparisons between observatories between 1951 and 1971, best consistency between the 10.7cm flux and observations at other wavelengths is obtained by multiplying the 10.7cm flux data by 0.9. Fluxes scaled in this way are designated URSI Series D. The history of the solar flux calibration process is reviewed by H. Tanaka (1973) of the Research Institute of Atmospherics, Nagoya University, as convener of then Comm. 5 of URSI.

Depending upon the level of activity and possibly the phase of the solar cycle, the fluxes contain contributions from active regions, areas of enhanced emission outside active regions, and a constant contribution from the quiet sun. The sources and emission mechanisms contributing to the 10.7 cm flux are discussed by K. F. Tapping (1987). The flux determinations sometimes contain contributions from transient events. Using empirical criteria these can be filtered from the data, although the degree to which they can be removed varies from example to example. However, a study by K. F. Tapping and D. P. Charrois (1994), suggests that in general the spot measurements are within 1% of the flux averaged over whole observing days after transient events have been eliminated.

The quiet sun level is the flux density which would be observed in the absence of activity. Extrapolation to zero of plots of the 10.7 cm flux against other activity indices such as plage area or total photospheric magnetic flux in active regions suggest a quiet sun flux density of about 64 s.f.u. This is rarely attained. Even at solar minimum there is usually some activity; the lowest observed fluxes are usually 65-67 s.f.u. The observed excess of the 10.7cm flux over the quiet sun level is known as the slowly-varying (S-) component.

A selected bibliography is given in Covington (1977).

These solar radio noise indices are published in accordance with a CCIR Recommendation originally from the Xth Plenary Assembly, Geneva, 1963 (maintained at XIth through XIVth Plenaries), which states "that the monthly-mean value of solar radio-noise flux at wavelengths near 10 cm should be adopted as the index to be used for predicting monthly median values of foE and foF1, for dates certainly up to 6, and perhaps up to 12 months ahead of the date of the last observed values of solar radio-noise flux."

Empirical formulas have been established for the relationship between F_10.7 and solar sunspot number. Accurate spot determinations of the 10.7 cm flux (actually a flux density) are made at local noon; previously 1700UT at Ottawa and now 2000UT at Penticton.

Auroral Electrojet index (AE)

1. it can be derived on an instantaneous basis or from averages of variations computed over any selected interval;
2. it is a quantitative index which, in general, is directly related to the processes producing the observed magnetic variations;
3. its method of derivation is relatively simple, digital, and objective and is well suited to present computer processing techniques;
4. it may be used to study either individual events of statistical aggregates.

Polar Cap Index (PC)

The PC-index is based on an idea by Troshichev et al. (1979) and developed in papers by Troshichev and Andrezen (1985), Vennerstrøm and Friis-Christensen (1989), Troshichev et al. (1988), Vennerstrøm (1991), and Vennerstrøm et al. (1991). Earlier data for 1975--1982 appear in Troshichev et al. (1991). The data from 1975 to 1993 are published in Report UAG-103 in graphical form. These data are published monthly in the Solar-Geophysical Data (SGD).

Data exists for the following time periods:
Thule: 1975--present: 15-minute data
Vostok: 1978--1979, 1983-1992: 15-minute data
Vostok: 1992--present: 1-minute data

Comprehensive Flare Index (CFI)

A: Originally the importance of ionizing radiation as indicated by the importance of associated SID, scale 1-3; but currently scaled from the x-ray flare class, class C being 1, class M being 2, and class X being 3.

B: Importance of H alpha flare; scale 1-3 (3 includes flare importance classes 3 and 4).

C: Log of 10.7 cm peak radio flux in units of 10^-22 W m^-2 Hz^-1.

D: Effects associated with the dynamic radio spectrum: Type II burst = 1, continuum storm = 2, Type IV burst = 3.

E: Log of 200-MHz flux in same units as C.

The CFI was devised and documented by Helen Dodson Prince and Ruth Hedeman at the McMath-Hulbert Observatory.

Archives

Bartels, J., N. H. Heck, and H. F. Johnston, The three-hour-range index measuring geomagnetic activity, J. Geophys. Res., 44, 411, 1939.

Bartels, J., The technique of scaling indices K and Q of geomagnetic activity, Ann. Intern. Geophys. Year 4, 215-226, 1957.

Bartels, J., The geomagnetic measures for the time-variations of solar corpuscular radiation, described for use in correlation studies in other geophysical fields, Ann. Intern. Geophys. Year 4, 227-236, 1957.

Chapman, S., and J. Bartels, Geomagnetism, Oxford at the Clarendon Press, London, 1940.

Clauer et al., Solar wind control of the low-latitude asymmetric magnetic disturbance field, J. Geophys. Res., 88, 2123-2130, 1983

Covington, A. E., Solar Radio Emission at 10.7cm, J. Royal Astron. Soc. Canada, Vol. 63, 125, 1969.

Covington, A. E., Algonquin Radio Observatory Report No. 5, A Working Collection of Daily 2800 MHz Solar Flux Values 1946-1976, Herzberg Institute of Astrophysics, N. R. C. of Canada, Ottawa, Canada, 1977.

Crooker, N.C. and G.L. Siscoe, A study of the geomagnetic disturbance field asymmetry, Radio Sci., 6, 495-501, 1971.

IATME Bulletin, 14, p. 320, resolution 9, 1954.

Iyemori, T.: Storm-time magnetospheric currents inferred from mid-latitude geomagnetic field variation, J. Geomag. Geoelectr., 42, 1249–1265, 1990

Iyemori, T., Araki, T., Kamei, T., and Takeda, M.: 1999, Mid-Latitude Geomagnetic Indices ASY and SYM, Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University

Kan, J. R., and L. C. Lee, Energy coupling function and Solar wind-magnetosphere dynamo, Geophys. Res. Lett., 6, 577-580, 1979.

Kawasaki, K. and S.-I. Akasofu, Low-latitude DS component of geomagnetic storm field, J. Geophys. Res., 76, 2396-2405, 1971

Mayaud, P. N., Indices Kn, Ks, Km (1964 - 1967), CNRS, Paris 1968.

Mayaud, P. N., A hundred year series of geomagnetic data 1868 to 1967, indices aa, storm sudden commencements, IAGA Bulletin 33 page 252, IUGG Publications Office, Paris 1973.

Mayaud, P. N., Derivation, Meaning, and Use of Geomagnetic Indices, Geophys. Monogr. Ser., Vol. 22, AGU, Washington, D. C., 1980.

McNish,A. G., and J. V. Lincoln, Trans. Am. Geophys. Union, 30, 673-685, 1949.

Menvielle, M., and A. Berthelier, The K-derived planetary indices: description and availability, Rev. Geophys., 29, 415-432, 1991. Correction: Ibid., 30, 91, 1992.

Papitashvili, V. O., L. I. Gromova, V. A. Popov, and O. Rasmussen, Northern Polar Cap magnetic activity index PCN: Effective area, universal time, seasonal and solar cycle variations, Scientific Report 01-01, Danish Meteorological Institute, Copenhagen, Denmark, 57 pp, 2001.

Rangarajan, G. K., Indices of magnetic activity, in Geomagnetism, edited by I.A. Jacobs, Academic, San Diego, 1989.

Siebert, M., Maßzahlen der erdmagnetischen Aktivit7auml;t, in Handbuch der Physik, vol. 49/3, 206-275, Springer, Berlin Heidelberg, 1971.

Siebert, M. and J. Meyer, Geomagnetic Activity Indices, in The Upper Atmosphere (Eds. W. Dieminger et al.), 887-911, Springer, Berlin Heidelberg, 1996.

Sugiura, M., Hourly values of equatorial D_st for the IGY, Ann. Int. Geophys. Year, 35, 49, 1964.

Sugiura, Masahisa, and T. N. Davis, Auroral electrojet activity index AE and its universal time variations, J. Geophys. Res., 71, 785-801, 1966.

Tanaka, H., et al., Absolute calibration of solar radio flux density in the microwave region, Solar Physics, Vol. 29, 243, 1973.

Tapping, K. F., Recent Solar Radio Astronomy at Centimeter Wavelengths: The Temporal Variability of the 10.7-cm Flux, J. Geophys. Res., Vol. 92, D1, 829-838, 1987.

Tapping, K. F., and D. P. Charrois, Limits to the Accuracy of the 10.7cm Flux, Solar Physics, 1994.

Troshichev, O. A., N. P. Ddmitrieva, and B. M. Kuznetsov, Polar cap magnetic activity as a signature of substorm development, Planet. Space Sci., 27, 217, 1979.

Troshichev, O. A., and V. G. Andrezen, The relationship between interplanetary quantities and magnetic activity in the southern polar cap, Planet. Space Sci., 33, 415-419, 1985.

Troshichev, O. A., V. G. Andrezen, S. Vennerstrøm, and E. Friis-Christensen, Magnetic activity in the Polar Cap - a new index, Planet. Space Sci., 36, 1095-1102, 1988.

Troshichev, O. A., V. G. Andrezen, S. Vennerstrøm, and E. Friis-Christensen, Polqr cap (PC) geomagnetic activity index for 1975-1982, World Data Center B, Soviet Geophysical Committee, Academy of Sciences of the USS, Moscow, 142 pp, 1991.

Vennerstrøm, S., The geomagnetic activity index PC, Ph. D. Thesis, Scientific Report 91-3, Danish Meteorological Institute, 105 pp, 1991.

Vennerstrøm, S., E. Friis-Christensen, O. A. Troshichev, and V. G. Andrezen, Comparison between the polar cap index, PC, and the auroral electrojet indices AE, AL, and AU, J. Geophys. Res., 96, 101-113, 1991.

Vennerstrøm, S., E. Friis-Christensen, O. A. Troshichev, and V. G. Andrezen, Geomagnetic Polar Cap (PC) Index 1975-1993, Report UAG-103, WDC-A for Solar-Terrestrial Physics, NOAA/NGDC, Boulder, Colorado, 274 pp, 1994.