Determining the Sharp, Longitudinal Gradients in Equatorial ExB Drift Velocities Associated with the

4-cell, Non-migrating Structures

David Anderson and Eduardo A. Araujo-Pradere

( Univ. of Colorado/CIRES and NOAA/SWPC)

Acknowledgements: Endawoke Yizengaw and Cesar Valladares (Institute for Scientific Research, Boston College)

Outline

• Observations of the 4-cell, non-migrating, longitude structures• TOPEX/TEC observations of the 4-cell pattern and magnetometer-inferred ExB drift velocities at one of the boundaries• Quantifying the sharp longitude gradients in ExB drift velocities at the cell boundaries using C/NOFS satellite observations • Modeling the daytime, equatorial ion densities associated with the sharp longitude gradients in ExB drift• LISN magnetometer-inferred ExB drift velocities observed in the Jicamarca and Alta Floresta longitude sectors• Summary and future work

Nighttime IMAGE 135.6 nm radiances from O+ radiative recombination (Immel et al., 2006)

Noontime, magnetometer-observed equatorial electrojet current density vs longitude from CHAMP, SAC-C and Oersted satellites (England et al., 2006)

IMAGE = Imager for Magnetopause-to-Aurora Global Exploration.

Diurnal, eastward propagating, non-migrating (DE3) tides are thought to originate in the troposphere through latent heat release and to be

responsible for the 4-cell, non-migrating structures observed in the equatorial ionosphere… Observations of the 4-cell structures

Upward ExB Drift Velocity vs Geographic Longitude from ROCSAT-1 Satellite Between 1000 and 1100 LT (Kil et al.,

2007)

Theoretically-calculated Vertical ExB Drift Velocity vs Longitude and Local Time from TIMEGCM Model

(England et al., 2010)

Solar Energy Budget (from Kerri Cahoy’s ISEA talk)

Possible Mechanism that Accounts for the 4-cell Structure – Tropical Latent Heat Release in the Troposphere from Kerri

Cahoy’s ISEA talk

Ground-based, Magnetometer-Inferred Daytime Vertical ExB Drift Velocities

Peruvian sector Philippine sector

Location of the Magnetometers

THIRUNELVELI (Geog. 8.70o N; 76.95o E Geomag. 0.46o S)

ALIBAG(Geog. 18.62o N; 72.87o E Geomag. 10o N)

Jicamarca (Geog. 11.92o S; 283.13o E Geomag. 0.8o N)

Piura (Geog. 5.18o S; 279.36° E Geomag. 6.8o N)

Davao (Geog. 7o N; 125.4o E Geomag. 1.32o S)

Muntinlupa (Geog. 14.37o N; 121.02o E Geomag. 6.39o N)

Yap (Geog. 9.3o N; 138.5o E Geomag. 0.5o N)

Biak (Geog. 1.08o S; 136.0o E Geomag. 9.74o S)

Philippine and Indonesian Sectors Indian SectorPeruvian Sector

Quantifying the Daytime, Equatorial ExB Drift Velocities Associated with the 4-cell, Non-migrating Tidal Structures

Sharp longitude gradients in ExB drift velocity are responsible for sharp gradients in TOPEX/TEC between the Philippine and Indonesian sectors

C/NOFS VEFI and IVM sensors will determine the longitude gradients in ExB drift velocities at each of the 4 cell boundaries

PHIL

IPPI

NES

IND

ON

ESIA

Local Time

E x

B dr

ift [m

/s]

Satellite and Payload

The Communication/Navigation Outage Forecast System (C/NOFS) satellite is in a 13 degree inclination orbit and the Ion Velocity Meter

(IVM) has been used to obtain the sharp longitude gradients in ExB drift velocities at the boundaries of the 4 cell structures, on a day-to-day basis

C/NOFS Ion Velocity Meter (IVM) Constraints

• IVM observations between 1000 and 1300 LT• Approximate LT window for maximum ExB drift velocity

• IVM observations below 500 km• Low enough altitude so that O+ is the major ion• Observations are averaged over each degree of longitude•Due to this averaging, sharp “spikes” in ExB drift may be introduced. These will be analyzed at a later time, on a case by case basis

• IVM observations for 2009• Primarily the months of October, March and December

PHIL

IPPI

NES

IND

ON

ESI

A

Longitude gradient in ExB drift ~ - 4 m/sec/degree

The steep longitude gradient in ExB drift velocity at the cell boundary in the Peruvian sector

PHIL

IPPI

NES

IND

ON

ESI

A

Longitude gradient in ExB drift ~ 1 m/sec/degree

All 3 days have equivalent slopes and they delineate the increase in ExB drift velocity across the Atlantic sector

The combination of upward, daytime ExB drift velocity perpendicular to B and downward diffusion parallel to B by gravity and pressure gradient forces create crests in ionization at +/- 15 to 18 degrees magnetic latitude known as the equatorial anomaly. If the daytime, ExB drift velocities are

significantly lower or are absent, then the crests in ionization are significantly closer to the magnetic equator or are absent

Low Latitude Transport Mechanisms

IVM-observed ExB Drift Velocity and Ion Density vs Geographic Longitude on March 23, 2009 in the Peruvian Longitude Sector

Geographic Longitude

Ion DensityExB Drift

Ion

Den

sity

[cm

-3]

ExB

Drift [m

/s]

EQUINOX

IVM-observed ExB Drift Velocity and Ion Density vs Geographic Longitude on Oct. 7, 2009 in the Atlantic Longitude Sector

Geographic Longitude

Ion DensityExB Drift

Ion

Den

sity

[cm

-3]

ExB

Drift [m

/s]

EQUINOX

The Global Ionosphere Plasmasphere (GIP) theoretical, time-dependent ionospheric model has been used to calculate ion densities as a function of

altitude, latitude, longitude and local time in the Peruvian and Atlantic longitude sectors to demonstrate the effects of sharp longitude gradients

in ExB drift velocities on calculated ion density distributions

GIP-calculated Ion Densities vs Geographic Latitude and Longitude at 400 km, 1400 LT Incorporating the Scherliess-Fejer Climatological ExB Drift Velocities x 1.5 (Control run)

GIP-calculated Ion Densities vs Geographic Latitude and Geographic Longitude at 400 km and 1400 LT Incorporating the IVM-observed ExB Drift Velocities between 280o and 340o Geog. Long, beginning at 10:00 LT

Comparing Magnetometer-inferred, Vertical ExB Drift Velocities Between Jicamarca and Alta

Floresta Longitude Sectors

• Critical to determine Alta Floresta’s dip latitude• We are making the assumption that it is within a degree of the dip equator• We assume a linear relationship between delta H values and ExB drift velocities• Choose JULIA 150 km echo observations of vertical ExB drift velocity vs local time for April 9, 2011• Compared the JULIA ExB drifts with the Jicamarca-Piura delta H values for April 9, 2011• Assuming a linear relationship, we determined that a linear slope of 0.31 m/sec/nT gave the best agreement between magnetometer-inferred ExB drift velocities and the JULIA observed ExB drift velocities• We applied this slope to the September, 2011 Jicamarca-Piura and the Alta Floresta-Cuiaba delta H values to determine ExB drifts

Delta H Comparisons Between Jicamarca and Alta Floresta Longitude Sectors for September 2 and 16

Assuming an ExB vs delta H linear slope of 0.31 m/sec/nT, the peak ExB drift velocity for Jicamarca-Piura is ~ 28 m/sec and for Alta Floresta-Cuiaba is ~6.5 m/sec.

On September 16, the peak ExB drift velocity is ~ 31 m/sec in the Jicamarca longitude sector and ~ 13 m/sec in the Alta Floresta longitude sector.

Delta H Comparison for a Geomagnetically Disturbed Day on September 17

Summary and Future Work

• Very sharp longitude gradients in daytime, vertical ExB drift velocities are observed by the C/NOFS IVM sensor at the boundaries of the 4-cell, non-migrating ionospheric structures• These sharp gradients occur on a day-to-day basis• Incorporating these ExB drift velocities into a theoretical, time-dependent ionospheric model produces sharp longitude gradients in calculated ion densities and TEC values• Sharp longitude gradients in ExB drifts are also observed from ground-based magnetometer measurements between the Jicamarca longitude sector (284 E. geog. long.) and the Alta Floresta longitude sector ( 304 E. geog. long.)• Do the LISN chains of GPS receivers observe sharp longitude gradients in observed TEC values?• Related to the 4-cell boundary between the Peruvian and Brazilian sectors, the major advantage of LISN is the ability to obtain continuous, day-to-day observations of TEC and ExB drifts to determine seasonal changes and whether these occur gradually or abruptly

Q & A

Ion DensityExB Drift

Ion

Den

sity

[cm

-3]

ExB

Drift [m

/s]