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Space Environment Definitions

Space weather National Space
Weather Plan
AU Solar Irradiance
F10 F81 Lya L81
Integrated solar
irradiance proxies
E10 E81 St or S_C
Qeuv Rsn Tinf Peuv
E1_40 I(t) E3h B3h
A3h E1s SRC Ap
a1s Historical Nowcast Forecast
High Time Resolution Model Upgrade &
Release scheme
  SY   versioning scheme

Generalized definitions:

Space weather.The shorter-term variable impact of the Sun’s photons, solar wind particles, and interplanetary magnetic field upon the Earth’s environment that can adversely affect our technological systems is colloquially known as space weather. It includes, for example, the effects of solar coronal mass ejections, solar flares, solar and galactic energetic particles, as well as the solar wind, all of which affect Earth’s magnetospheric particles and fields, geomagnetic and electrodynamical conditions, radiation belts, aurorae, ionosphere, and the neutral thermosphere and mesosphere.

National Space Weather Implementation Plan. The National Space Weather Program (NSWP) Implementation Plan (IP), second edition (FCM-P31-2000) published in July 2000 and accessible at, describes the goal to improve our understanding of space weather effects upon terrestrial systems. Operationally characterizing space weather as a coupled, seamless system from the Sun to Earth is one achievement of this goal. Among the areas of interest for improved understanding are the space weather processes affecting the thermosphere and ionosphere.

AU Designates an Astronomical Unit (AU) and is a unit of length equal to the mean distance between the Sun and Earth. The accepted value is 149,597,870,660(20) meters. Distances between objects within the solar system are frequently expressed in terms of AU.

Solar irradiance is the Sun’s radiation integrated over the full disk of the Sun expressed as a unit of power (Watts) through a unit of area (per meter squared) centered at a specified wavelength and integrated across a wavelength interval.

Solar Irradiance Platform input proxy definitions:

F10 is the daily value of the 10.7-cm solar radio emission measured by the Canadian National Research Council Dominion Radio Astrophysical Observatory at Penticton, BC, Canada ( The “observed” value is the number measured by the solar radio telescope at the observatory, is modulated the level of solar activity and the changing distance between the Earth and Sun, and is the quantity to use when terrestrial phenomena are being studied. When the Sun is being studied, it is useful to remove the annual modulation of F10 by the changing Earth-Sun distance and the “1 AU adjusted” value is corrected for variations in the Earth-Sun distance, giving the average distance. Penticton measures the F10, NOAA/SEC reports the F10, and numerous organizations, including SET, forecast the F10. Its units are solar flux units (sfu) or x10-22 Watts per meter squared per Hertz. Normal practice is to refer to the value as “F10.7” but F10 is used here as an abbreviation.

F81 is the daily value of the 81-day running average of the F10 centered at the current epoch (date) and in the F10 units.

Lya is the daily value of the solar hydrogen atom emission of Lyman-alpha irradiance at 121.67 nm measured from outside the atmosphere and reported in photon flux of x10+09 photons per centimeter squared per second.

L81 is the daily value of the 81-day running average of the Lya centered at the current epoch (date) and in the Lya units.

Solar Irradiance Platform proxies definitions:

Integrated solar irradiance proxies. With the release of Solar Irradiance Platform v2_32, there are a number of seven integrated flux irradiance proxies that are produced for the benefit of specific user communities. These proxies are provided in addition to the three spectral irradiance wavelength formats of 1 nm bins for the full spectrum from 1–1,000,000 nm, 39 EUV wavelength groups/lines from 1–105 nm, and 867 EUV lines from 1–122 nm. Each wavelength format is reported in three flux formats of energy (ergs per centimeter squared per second), photon (photons per centimeter squared per second), and SI units (Watts per meter squared). The seven proxies are described in more detail below.

E10 is the daily value of the integrated solar extreme ultraviolet (EUV) energy flux from1–105 nm at the top of the atmosphere and reported in F10 units. It represents the spectral solar energy available for photoabsorption and photoionization that is separately input into numerical models. Normal practice is to refer to the value as “E10.7” but E10 is used here as an abbreviation. See the PDF document: "E10 comparison with F10 for satellite operations".

E81 is the daily value of the 81-day running average of the E10 centered at the current epoch (date) and in the E10 units.

S(t) or S_C is the daily value of the integrated solar spectrum in units of Watts per meter squared and is provided to users who require the integrated spectrum variability. In early versions of the SOLAR2000 model (v1.yz), the variability comes from the solar spectrum between 1–122 nm (EUV variability). Longward of 122 nm in the v1.yz model, the ASTM E490 solar reference spectrum is used. Hence, the current variability in S is not the same as the total solar irradiance (TSI). In upgrades beyond v1.yz of SOLAR2000, time-varying spectral models are included to represent the ultraviolet, visible/infrared, and theoretical spectral variability in versions 2.yz, 3.yz, and 4.yz, respectively. In v3.yz, this spectrum will be extremely useful for space systems’ users who require an operational, variable integrated solar spectrum for solar radiation pressure calculations on spacecraft. In v4.yz, a high spectral resolution of the Sun’s irradiances will be provided for use in satellite imagery calibration.

Qeuv is the daily value of the thermospheric heating rate derived from an analysis of the time-dependent solar heating of the thermosphere as a function of EUV energy by wavelength, altitudinal heating efficiency, unit optical depth, absorption cross section of each neutral species, and density of each species. These combined quantities are the constituent volume-heating rate in the thermosphere and are integrated across all species, wavelengths, and altitudes for a unit of time to become the derived total thermospheric heating rate in ergs per centimeter squared per second. A third degree polynomial fit is made between the total heating rate and E10.7 for several years over a solar cycle and this is the Qeuv.

Rsn is the daily value of the derived sunspot number for use in ray-trace algorithms that historically use the Wolf sunspot number, Rz. Rsn is dimensionless and is derived from a third degree polynomial fit between Rz and E10.7 for several years over a solar cycle. Rsn differs from Rz during solar maximum conditions and does not reach the highest values of Rz. We believe it provides a capability for more accurately representing the variations in the ionosphere that come directly from solar EUV photoionization.
Tinf is the daily value of the Earth exospheric temperature at 450 km in units of Kelvin (K). It was developed using a first-principles thermospheric model and is useful for long-term studies to investigate potential anthropogenic climate change effects (cooling) in the thermosphere and subsequent changes to the ionospheric E and F2 layer heights. Tinf is derived from a third degree polynomial fit between the first principles derived exospheric temperature and E10.7 for several years over a solar cycle.

Peuv is the daily value of the EUV hemispheric power in units of Watts and is complementary to the auroral hemispheric power index. It is designed for science research and operations use. It is derived from the solar EUV energy flux summed across all wavelengths from 1–105 nm. This value is approximately 6 ergs per centimeter squared per second for an average level of solar activity. This solar energy is assumed to be input across the disk of Earth and is reported in units of gigaWatts (GW). The Peuv heating is greater than auroral hemispheric power except during storm periods.

E1_40 is the daily value of the integrated EUV energy flux between 1-40 nm in units of ergs per centimeter squared per second.

I(t) is the daily value of the solar spectral irradiances in 1nm, 39 wavelength groups, and 867 EUV wavelength bins. The irradiances are created using the F10 coronal proxy (either the observed or adjusted values) and the Lyman-alpha chromospheric proxy which is a UARS-based composite five solar cycle set through June 23, 2001 followed by the NOAA 16 Mg II core-to-wing ratio data (Mg II) scaled to Lyman-alpha. The Mg II FTP data is available at

E3h is the MFD bulletin 3-hour average value of the E10 forecast to 72 hours in E10 units.

B3h is the MFD bulletin 3-hour average value of the E81 forecast to 72 hours in E10 units.

a3h is the MFD bulletin 3-hour average value of the Ap forecast to 72 hours in Ap units.

E1s is the MFD bulletin 1-sigma uncertainty of E3h in E10 units.

SRC is the MFD bulletin data source designation (Issued, Nowcast, Predicted).

Ap is the daily mean value of the planetary geomagnetic index in units of 2 nanoTesla (nT).

a1s is the MFD bulletin 1-sigma uncertainty of a3h in Ap units.

Historical. SOLAR2000 daily irradiances and integrated irradiance proxies are provided for all applications from research to operational systems starting from February 14, 1947 through the current date.

Nowcast. SOLAR2000 nowcast irradiances and integrated irradiance proxies, using the operational NOAA 16-18  SBUV Mg II data for the chromospheric proxy and the 20 UT observed F10 for the coronal proxy, are provided hourly by the SOLAR2000 Operational Grade model located at NOAA Space Environment Center (SEC) in Boulder, Colorado ( and by the SET proprietary server ( The model is run independently and hourly at both sites. Although the information content changes only twice per day in 2003 using the daily 20 UT F10 and the daily Mg II (NOAA 16), or a few times per day (NOAA 16 combined with NOAA 17 starting in late 2003), the cadence will significantly increase with the inclusion of 5-minute data using the GOES-N EUV broadband detector data after 2005. After that time, the F10 and Mg II will be retained as redundant input proxy data to ensure a capability of calculating the irradiances. At that time, the GOES-N data, absolutely calibrated to the TIMED/SEE instrument data, will become the primary data set for the EUV part of the spectrum. The Mg II will still remain the primary data set for calculating the FUV irradiances after 2005. In addition to graphical representations of the irradiances located at the web sites above, nowcast data files are located and updated with the same hourly cadence at SEC’s anonymous FTP server The files located at that site of “E10.7 nowcast data,” “Solar spectral data,” and “Validation of today’s E10.7 data” provide the nowcast E10 with ±1-sigma values, the full solar spectrum at 1 nm resolution, and comparative nowcast data of F10, F81, Lya, L81, E10, E81, and S.
The definition of nowcast has evolved in current operations to indicate the period of time between –24 hours to the current epoch (time). Starting 24 hours in the past, the input parameters required for model runs, i.e., the F10 and Mg II data, are already operationally issued and will not change. However, at the current epoch, or “0” hour, the solar conditions will have changed slightly and new information has not yet been received to precisely define what the new proxy values are. Hence, an estimate made of the current conditions and the interpolation from known to unknown conditions during the past 24-hours constitutes a nowcast.

Forecast. Forecast irradiances and integrated irradiance proxies are provided for government and commercial customers. The Solar Irradiance Platform SY version models current (and first) generation forecast algorithm is denoted FGen 1x and relies on linear predictive techniques. The fundamental assumption of persistence in solar irradiances at time scales of interest (3-days, 14-days, 28-days, 4-months, 1 solar cycle, and 5 solar cycles) is the basis for these techniques. FGen 2 will provide forecast irradiances on the same timescales based on physics, measurements, and mathematical tools.

High time resolution.In FGen 1x, the forecasts for next 72-hours are produced on a 3-hour cadence and synchronized with the release of the NOAA/SEC and U.S. Air Force Kp and ap geomagnetic indices.

Upgrade and Release Convention. Regular and continuous upgrades to the Solar Irradiance Platform are occurring during the first half of the decade starting in 2000. These upgrades include additional spectral range variability (FUV, UV, VIS, IR), enhanced accuracy with the inclusion of new datasets and improved proxy regression algorithms, improved specification of the uncertainty in the irradiances, the development of nowcast and forecast irradiances along with the historical representations, and the development of new integrated irradiance proxies for user communities. The model has been upgraded 18 times between October 7, 1999 (v0.10) and May 2, 2003 (v2.21) through the publicly released SOLAR2000 Research Grade model. The convention for the model Grade designation is explained in detail above and in briefly summarized here:

RG Research Grade provides daily historical to current epoch data through a platform-independent IDL© GUI application.

PG Professional Grade provides daily historical to current epoch data through a platform-independent IDL© GUI application, but with forecast capabilities not available with the RG version.

SY System Grade provides historical, nowcast, forecast data in all time resolutions as a turn-key system on at a user-specified location.