The Challenges of Validating Global Assimilative Models of the Ionosphere L.F. M c Namara 1,C.R. Baker 2, G.J. Bishop 2, D.T. Decker 2, J.A. Welsh 2 1. Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA 2. Space Weather Center of Excellence, Air Force Research Laboratory, Hanscom Air Force Base, MA, USA

Overview AFRL has been tasked by AFWA to validate USU-GAIM, its operational global ionospheric model. AFRL is validating V2.4.3 of the Gauss-Markov Kalman Filter version of USU-GAIM. Validation requires suitable Ground-Truth observations. We are concerned here with situations in which the ground-truth contains evidence of phenomena that are not currently modeled by GAIM.

CHAMP Electron Densities The CHAMP observations of the electron density at an altitude of ~400 km are the main source of ground- truth data. We work in terms of plasma frequency. Electron density N = 1.24x10 10 fn 2, where fn is the plasma frequency in MHz, and N is in m -3.

Equatorial Plasma Bubbles Equatorial Plasma Bubbles (EPB) are banana-shaped field-aligned depletions in electron density that sometimes occur in the nighttime equatorial F region. EPBs are caused by instabilities in the base of the nighttime F layer during its rapid post-sunset rise. The global GAIM model does not have sufficient resolution to generate the steep gradients that trigger the instabilities. EPBs can be removed fairly well from the CHAMP observations, so validation is not a problem.

N-Node Waves The equatorial values of the CHAMP plasma frequency at a fixed local time have been found to have a 4-node variation, with the nodes being separated by 90 o in longitude. Other N-node variations have also been found. The variations have their source in non-migrating tides in the neutral atmosphere, which increase the conductivity of the E layer at different longitudes. This leads to increased plasma frequencies in the anomaly peaks (and decreases at the equator).

Median CHAMP Plasma Frequencies 20 LT Longitude / Dip latitude variation of the median CHAMP plasma frequency, in lat/lon pixels, days , 2004, 2000 LT (Aug/Sep). CHAMP electron density variations at the anomaly peaks have a 4-node structure, and are up to 3x

N-node Structure in IFM/GAIM GAIM V2.4.3 uses the Ionospheric Forecast Model (IFM) as its background model. The IFM does not show any N-node structure. The GMKF model is the result of assimilating real-time ionospheric observations. The GMKF plasma frequencies do not show any clear evidence of any N-node structure when only GPS TEC observations are assimilate.

N-node Structure in IFM/GAIM Replacing the vertical drift model by one based on ROCSAT observations should endow the IFM global specification with the required N-node structure. It is not known if the assimilated GPS TEC would erase this structure. More work is need to investigate any longitudinal structure in the slant GPS TEC observations The vertical TEC observations from TOPEX have amplitude fluctuations of only 20%. The CHAMP electron density (~400 km) has factors of two min-max variation.

SSUSI UV radiances GAIM will assimilate the DMSP/SSUSI radiances. For DMSP/F16, the evening passes are at ~20 LT. The disk radiances for September 2004, 20 LT, show a 4-node structure. When GAIM assimilates these radiances, the GAIM plasma frequencies also show a 4-node structure. Thus GAIM can legitimately be validated against CHAMP.

UV Radiances from SSUSI The SSUSI disk radiances show a 4-node structure. September 2004, 20 LT. The max/min ratio often exceeds 2x. Curves are for the northern and southern anomalies.

GAIM Plasma Frequency When the SSUSI radiances are assimilated, the GAIM plasma frequencies also show a 4-node structure, with minima at 60 o, 150 o, and 240 o. The expected minimum at ~330 o is problematic.

Summary EPBs can be accounted for, and so cause few validation problems. Validations of GAIM against CHAMP ground-truth data are legitimate when UV radiances are assimilated – both the GAIM specification and the CHAMP ground- truth data exhibit an N-node structure. If observations that do not show an N-node structure are the only ones assimilated, the GAIM specification will not show such structure, and the validations against CHAMP would not be legitimate.

For the Future The IFM could be modified to use a model of the vertical drift that includes an N-node structure. We really do not know at this stage if the available slant TEC observations from GPS sites exhibit an N- node structure. If they do not, will they swamp any other types of data that do show the structure? Will they swamp the effects of a longitudinal structure in a modified vertical drift model?

Evidence of EPBs Plasma frequencies for multiple CHAMP passes through the equatorial ionosphere. The very low values between +10 and -10 dip latitude are mostly evidence of EPBs. The plasma frequency is highest at the anomaly peaks (~12 MHz). Observed and GAIM values of the plasma frequency at the location of CHAMP for one evening pass. The CHAMP values drop towards zero between +10 and -10 dip latitude. The GAIM values do not. The different GAIM values correspond to different data being assimilated. EPB

Errors due to EPBs The positive errors between +10 and -10 dip latitude indicate the presence of EPBs. Most of the positive errors near the dip equator disappear when the EPBs are excised (crudely).

Median CHAMP Plasma Frequencies 08 LT Longitude / Dip latitude variation of the median CHAMP plasma frequency, in lat/lon pixels, days , 2001, 08 LT (Jan/Feb) CHAMP electron density at the anomaly peaks is much higher over the Pacific. There is no 4- node structure.

N-Node Waves The equatorial values of the CHAMP plasma frequency at a fixed local time have been found to have a 4-node variation, with the nodes being separated by 90 o in longitude. Other N-node variations have also been found. The variations have their source in non-migrating tides in the neutral atmosphere, which increase the conductivity of the E layer at different longitudes. The increased conductivity of the E layer leads to larger values of the equatorial vertical ExB drift. This leads to increased plasma frequencies in the anomaly peaks (and decreases at the equator).