TIMED/GUVI

The NASA satellite TIMED (Thermosphere, Ionosphere, Mesoshere Energetics and Dynamics) carries four instruments for measuring solar extreme ultraviolet (EUV) irradiances, winds, temperature, auroral energy inputs, and composition.   One of these is a far ultraviolet spectrographic imaging instrument, GUVI (Global Ultraviolet Imager), very similar in design to the DMSP SSUSI instrument.   CPI's involvement in GUVI is through algorithm development, and Dr. Douglas Strickland's participation on the GUVI science team as a co-investigator.   Additional support has come through a TIMED/CEDAR grant to Dr. James Hecht of the Aerospace Corporation whose focus has been comparison of auroral data products from coincident ground-based and GUVI measurements. Key data products come from CPI remote sensing algorithms similar to those previously developed for the SSUSI instrument (dayglow algorithms; auroral algorithms).   Products include:

• Characteristic energy E_o (keV) and energy flux Q (mW m^-2) of precipitating auroral electrons
• Simultaneously derived auroral O/N₂ (column density ratio referenced to the fixed N₂ value of 10^-17 cm^-2)
• O/N₂ derived from dayglow data
• Q_EUV (integrated measure of solar EUV/XUV energy flux shortward of 45 nm in mW m^-2)
  - Non-flare
  - Flare (separate algorithm)

The following papers address these quantities with contributions from one or more CPI technical staff members:

The above 2004 O/N₂ paper (A01302) addresses seasonal effects between hemispheres under geomagnetically quiet conditions (Figure 1 below) and the sensitivity of O/N₂ to LBH cross section uncertainties and to solar EUV/XUV scalings shortward of 20 nm.

Key results from the above JGR Q_EUV paper (L03801) include the variation of Q_EUV over the mid-July to early-August 2003 solar rotation (Figure 2 below), effect of solar flares on Q_EUV, excellent agreement with SOHO/SEM solar EUV data in a relative sense over the rotation (include spikes due to flares), and the sensitivity of Q_EUV to LBH cross section uncertainties and to solar EUV/XUV scalings shortward of 20 nm (similar to the sensitivity of O/N₂ addressed in the above paper).

CPI provided the GUVI O/N₂ results appearing in the above Meier et al. and Crowley et al. papers. These include the spectacular variations on Nov 20-21 2003 during a major geomagnetic storm from that period (Figure 3 below). The reduced O/N₂ over very extended geographical regions arose from atmospheric heating within the auroral oval over many hours followed by transport of heated (and compositionally disturbed air) in the midnight sector to lower latitudes by winds arising from the storm. It was then observed by GUVI many hours later on the dayside.

In an investigation described in detail by Strickland et al. [2007], we are using GUVI 135.6 [134.3 - 137.7 nm] and LBH_S [141.0 - 152.8 nm] dayglow measurements recorded during solar flares to estimate the spectrum of solar EUV/XUV irradiances. An entirely new algorithm has been developed beyond that documented in the above Q_EUV paper. It relies on TIMED/SEE measurements longward of 27 nm and derives the spectrum at shorter wavelengths with the constraint that the full spectrum replicate the coincident GUVI measurements. Derived spectra are tested by using them to produce E-layer electron density profiles (EDPs) with the AURIC model followed by comparisons with measured EDPs. Overall good agreement is achieved for solar flares addressed so far (X-class flares on Oct 28 and Nov 4 2003). Figure 4 below is taken from the 2007 paper and shows the significant increase in dayglow during these flares and other information as discussed in the caption.

There is also an active investigation directed at auroral remote sensing using the three GUVI spectral channels 135.6, LBH_S, and LBH_L [167.2 - 181.2 nm] from which E_o, Q, and O/N₂ are being derived. Much of the work has been in collaboration with J. Hecht of the Aerospace Corporation who is operating ground-based photometer systems at Poker Flat and Ft. Yukon, AK whose data are also being used to derive the above auroral products. Interest is in the comparison of products from coincident data. GUVI products for many passes through the auroral oval (Northern Hemisphere) have been produced in image form and as plots from cuts through these images. Figure 5 provides an example of GUVI auroral data and data product images from Oct 29 2003, a day of highly elevated geomagnetic activity. A paper is in preparation using results such as these that focus on comparisons between products from Hecht's data and from coincident GUVI data.

Figure 1. (Plate 1 from Strickland et al., J. Geophys. Res., 109, A01302, doi:10.1029/2003 JA010220, 2004) GUVI and NRLMSIS O/N₂ on days 76 (3/17/02), 196 (7/15/02), 264 (9/21/02), and 16 (1/16/03). Quiet geomagnetic conditions prevail on these days for which daily K_p is less than two. The time scales illustrate the fact that ~24 hrs of data were used to produce the GUVI images. Local times throughout the images at low latitudes are near noon. The selected dates serve to illustrate O/N₂ behavior under near-equinox and near-solstice conditions.

Figure 2. (Figure 4 from Strickland et al., Geophys. Res. Lett., 31, L03801, doi:10.1029/ 2003GL 018415, 2004) GUVI Q_EUV values are compared in a) with a linear parameterization of SOHO SEM central channel data. SEM and GOES X-ray data are shown at 5-minute time resolution in b).

Figure 3 (above and to the right). O/N₂ during the super storm that commenced on Nov 20 2003. Local time throughout the image is near noon. The time axis illustrates that 24 hours of data were used beginning mid-day on Nov 20. The dark regions contain disturbed air that is strongly deficient in atomic oxygen. Such regions remain in this state for many hours and contain a seriously depleted ionosphere (referred to as a negative ionospheric storm).